fix(desktop): surface a missing DATABRICKS_HOST instead of loading models forever (#8504)

## Summary

If the `databricks` / `databricks_v2` provider is selected but
`DATABRICKS_HOST` isn't set, the Model field stays on "Loading
models..." forever and never shows an error.

How it happens:

1. Desktop discovers Databricks models in-process only when
`DATABRICKS_HOST` is set. Without it, discovery falls back to `buzz-acp
models`. buzz-agent then fails at config time with `DATABRICKS_MODEL
required` (model is checked first) or `DATABRICKS_HOST required`.
2. `formatModelDiscoveryErrorStatus` maps those errors, and
`BUZZ_AGENT_PROVIDER is required`, to a `null` status. The idea was that
the field's Required row would explain the problem.
3. `deriveModelDiscoveryPending` treats "no data and no status" as still
pending. So a failed discovery with a `null` status looks like it is
still loading.

The fix:

- `formatModelDiscoveryErrorStatus` now always returns a status, and its
return type is no longer nullable. A failed discovery therefore always
settles.
- A missing Databricks host shows: "Set DATABRICKS_HOST to your
Databricks workspace URL under Advanced to load Databricks models."
- A missing provider shows a muted "Choose a provider to load models."

Builds that bake `DATABRICKS_HOST` in never reach this path.

### Screenshots

Settings → Agents → Agent defaults, with Buzz Agent + Databricks v2
selected, no `DATABRICKS_HOST`, and model discovery failing with
`DATABRICKS_MODEL required`. Captured from the desktop e2e build with
the mock bridge. The only difference between the two builds is
`personaModelDiscoveryStatus.ts`.

| Before | After |
|---|---|
| ![Before: Model field stuck on Loading
models](https://raw.githubusercontent.com/block/buzz/bbdfc1f8eb91d8f0f5dc6fd98567bc6af62d8117/pr-8504--before.png)
| ![After: Model field asks for
DATABRICKS_HOST](https://raw.githubusercontent.com/block/buzz/bbdfc1f8eb91d8f0f5dc6fd98567bc6af62d8117/pr-8504--after.png)
|

### Related issue

None found. Searched open PRs that touch `personaModelDiscoveryStatus` /
`usePersonaModelDiscovery`, and issues mentioning "loading models".

### Testing

- New jsdom test drives the real `usePersonaModelDiscovery` hook with a
rejected `DATABRICKS_MODEL required` discovery. It asserts that loading
settles and that the status names `DATABRICKS_HOST`. On `main` the test
fails (`modelDiscoveryLoading` stays `true`), and it passes with this
change.
- Updated unit tests in `personaModelDiscoveryStatus.test.mjs`.
- `pnpm test` in `desktop/` passes (6834 + 161 tests). `tsc --noEmit`
and biome are clean.
- Exercised in the desktop e2e build with the mock bridge (screenshots
above). Not yet exercised in the real Tauri app.

Signed-off-by: Fizz <6be7be23ec5bba868f3e1db5ec86013d76bd1a293dce5b435a192158fec13641@buzz.block.builderlab.xyz>
Co-authored-by: Fizz <6be7be23ec5bba868f3e1db5ec86013d76bd1a293dce5b435a192158fec13641@buzz.block.builderlab.xyz>

desktop/src/features/agents/ui/personaModelDiscoveryStatus.test.mjs

@@ -60,13 +60,30 @@ test("Buzz shared compute names a missing relay member roster", () => {
   assert.doesNotMatch(status?.message ?? "", /relay connection/);
 });
 
-test("model discovery status stays quiet for missing Databricks defaults", () => {
+for (const error of [
+  "config: DATABRICKS_HOST required",
+  "buzz-acp models failed (exit 1): config: DATABRICKS_MODEL required",
+]) {
+  test(`missing Databricks host is named, not swallowed: ${error}`, () => {
+    const status = formatModelDiscoveryErrorStatus(
+      new Error(error),
+      "databricks_v2",
+    );
+
+    assert.equal(status.tone, "warning");
+    assert.match(status.message, /DATABRICKS_HOST/);
+    assert.match(status.message, /Advanced/);
+  });
+}
+
+test("missing provider asks the user to choose one", () => {
   const status = formatModelDiscoveryErrorStatus(
-    new Error("config: DATABRICKS_HOST required"),
-    "databricks",
+    new Error("config: BUZZ_AGENT_PROVIDER is required"),
+    "",
   );
 
-  assert.equal(status, null);
+  assert.equal(status.tone, "muted");
+  assert.match(status.message, /Choose a provider/);
 });
 
 test("Databricks sign-in-required is a muted note pointing at the picker and CLI", () => {

desktop/src/features/agents/ui/personaModelDiscoveryStatus.ts

@@ -45,7 +45,7 @@ export function formatModelDiscoveryErrorStatus(
   error: unknown,
   provider: string,
   agentLabel?: string,
-): PersonaModelDiscoveryStatus | null {
+): PersonaModelDiscoveryStatus {
   const message = errorMessage(error);
 
   if (provider.trim() === "relay-mesh") {
@@ -116,12 +116,21 @@ export function formatModelDiscoveryErrorStatus(
     };
   }
 
+  // Desktop discovers Databricks models in-process whenever DATABRICKS_HOST
+  // is set, so either error from the harness fallback means the host is missing.
   if (
     message.includes("DATABRICKS_HOST required") ||
-    message.includes("DATABRICKS_MODEL required") ||
-    message.includes("BUZZ_AGENT_PROVIDER is required")
+    message.includes("DATABRICKS_MODEL required")
   ) {
-    return null;
+    return {
+      message:
+        "Set DATABRICKS_HOST to your Databricks workspace URL under Advanced to load Databricks models.",
+      tone: "warning",
+    };
+  }
+
+  if (message.includes("BUZZ_AGENT_PROVIDER is required")) {
+    return { message: "Choose a provider to load models.", tone: "muted" };
   }
 
   // Databricks transparent auth (agent_models_databricks.rs). The backend

desktop/src/features/agents/ui/usePersonaModelDiscovery.jsdom-test.mjs

@@ -1,7 +1,6 @@
 /**
- * The real discovery hook passes the selected runtime to the label code:
- * switching Claude Code -> Codex -> Claude Code relabels the same cached
- * discovery response.
+ * Exercises the real discovery hook: runtime switches relabel the same cached
+ * response, and a failed discovery always settles instead of loading forever.
  */
 import assert from "node:assert/strict";
 import { afterEach, test } from "node:test";
@@ -15,10 +14,11 @@ const discovered = {
   models: [{ id: "claude-sonnet-5", name: "Sonnet", description: null }],
 };
 const calls = [];
+let discover = () => Promise.resolve(discovered);
 const tauriMock = {
   invoke(command) {
     calls.push(command);
-    if (command === "discover_agent_models") return Promise.resolve(discovered);
+    if (command === "discover_agent_models") return discover();
     return Promise.reject(new Error(`unmocked Tauri command: ${command}`));
   },
   transformCallback() {
@@ -48,8 +48,15 @@ let container;
 afterEach(() => {
   act(() => root?.unmount());
   container?.remove();
+  discover = () => Promise.resolve(discovered);
 });
 
+function mountRoot() {
+  container = document.createElement("div");
+  document.body.appendChild(container);
+  root = createRoot(container);
+}
+
 let labels = null;
 function Probe({ selectedRuntime }) {
   const { discoveredModelOptions } = usePersonaModelDiscovery({
@@ -71,9 +78,7 @@ async function render(id) {
 }
 
 test("hook relabels discovered models when the runtime switches", async () => {
-  container = document.createElement("div");
-  document.body.appendChild(container);
-  root = createRoot(container);
+  mountRoot();
 
   await render("claude");
   assert.deepEqual(labels, [
@@ -92,3 +97,34 @@ test("hook relabels discovered models when the runtime switches", async () => {
     1,
   );
 });
+
+test("a missing Databricks host settles discovery instead of loading forever", async () => {
+  discover = () =>
+    Promise.reject(
+      "buzz-acp models failed (exit 1): config: DATABRICKS_MODEL required",
+    );
+  let state = null;
+  function DatabricksProbe() {
+    state = usePersonaModelDiscovery({
+      envVars,
+      isCustomProviderEditing: false,
+      modelFieldVisible: true,
+      open: true,
+      provider: "databricks_v2",
+      selectedRuntime: runtime("buzz-agent"),
+    });
+    return null;
+  }
+  mountRoot();
+
+  await act(async () => {
+    root.render(React.createElement(DatabricksProbe));
+  });
+  // databricks_v2 discovery is debounced before the IPC call.
+  await act(async () => {
+    await new Promise((resolve) => setTimeout(resolve, 400));
+  });
+
+  assert.equal(state.modelDiscoveryLoading, false);
+  assert.match(state.modelDiscoveryStatus?.message ?? "", /DATABRICKS_HOST/);
+});