feat(desktop): add databricks v1+v2 model discovery via buzz-agent lib

Implements in-process Databricks model catalog discovery (Option C) so
PKCE token-cache hits are covered without spawning a subprocess.

buzz-agent changes:
- catalog.rs: new pub module with discover_databricks_models(cfg) —
  v1 (serving-endpoints) and v2 (ai-gateway/v2/endpoints, paginated)
  HTTP fetch + parse helpers; 7 unit tests
- auth.rs: bearer_no_browser() trait default + PkceOAuthTokenSource
  override that stops before the browser step (returns Err(LlmAuth)
  when cache is empty)
- config.rs: Config::for_discovery() minimal constructor — avoids
  requiring DATABRICKS_MODEL/other fields irrelevant to discovery
- llm.rs: build_token_source() pub(crate) for catalog.rs access
- lib.rs: re-exports discover_databricks_models, ModelEntry,
  DATABRICKS_V2_KNOWN_MODELS, Provider, AgentError

desktop changes:
- Cargo.toml: buzz_agent_pkg path dep added
- agent_models.rs: discover_databricks_models wrapper calls
  buzz_agent_pkg::discover_databricks_models directly; LlmAuth ->
  Ok(None) fallthrough; other errors redacted; ~270 lines removed
- agent_models_tests.rs: stale parse/URL tests removed (moved to
  catalog::tests); is_databricks_provider test retained
- check-file-sizes.mjs: TEMP override lowered from 1244 -> 1066

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

desktop/scripts/check-file-sizes.mjs

@@ -132,6 +132,11 @@ const overrides = new Map([
   ["src/shared/ui/markdown.tsx", 2082],
   ["src/shared/ui/VideoPlayer.tsx", 2199],
   ["src/shared/ui/sidebar.tsx", 1042],
+  // Option C databricks-model-discovery: parse/HTTP logic moved to buzz-agent
+  // catalog module; agent_models.rs retains the thin wrapper (~50 lines).
+  // File still exceeds 1000 due to OpenAI/Anthropic discovery + subprocess
+  // fallback. Queued to split into dedicated discovery modules.
+  ["src-tauri/src/commands/agent_models.rs", 1066],
 ]);
 
 await runFileSizeCheck({