test(mobile): run push tests without a gateway dart-define (#9423)

A plain `cd mobile && flutter test` failed 11 push and notification
tests on a clean checkout of main, while CI passed. Agents running the
full suite kept reporting them as "pre-existing, also fails on main",
which hides real regressions in the same files.

### Root cause

Push is gated on the compile-time `BUZZ_PUSH_GATEWAY_URL` define
(`Env.pushGatewayConfigured`, since [#7261 configurable
gateway](https://github.com/block/buzz/pull/7261) made the gateway
optional). Without it the app is a push-less build:
`readBuzzPushEndpointGrants()` returns no grants and
`buzzPushLifecycleEnabled()` is false. CI's mobile shards, `just
mobile-test` and the lefthook pre-push lane (which runs `just
mobile-test`) all pass
`--dart-define=BUZZ_PUSH_GATEWAY_URL=https://push.example`; a bare
`flutter test` passes nothing. Each of the 11 was checked against that:

- `notification_toggle_test` (5 cases) and
`push_lease_revocation_outbox_test` "hosted relay assertion": no
endpoint grants without a gateway (`Bad state: No element` at the grant
lookup, `No endpoint grant exists for push lease tombstone`).
- `push_bridge_test` "reads and exposes persisted endpoint grants" and
"debug enrollment carries the configured relay and gateway URLs": the
bridge returns `[]` before calling native (`Expected: an object with
length of <1>, Actual: []`).
- `push_bootstrap_test` "default push requires relay capability" and
"new community starts registration": the lifecycle is disabled, so
registration never starts (`Expected <1>, Actual <0>`).
- `push_bridge_test` "development push gateway matches the compiled
configuration": a stale test. It expected a `https://push.buzz.xyz`
default that [#7261 configurable
gateway](https://github.com/block/buzz/pull/7261) removed from the app;
under CI's define both sides equal `push.example`, so it could never
fail there.

The same files pass on the same Mac with the define, so nothing is
environmental. The `Unexpected native call
takePendingNotificationResponse` log is unrelated noise: the toggle
test's native mock didn't answer that call, which logs (and is caught)
in CI too.

### Fix

Tests get the gateway from a test hook instead of the define.
`Env.pushGatewayUrl` becomes a getter over the compiled value with a
`@visibleForTesting` `debugPushGatewayUrlOverride` (the same pattern as
`debugUriStreamOverride`), and `test/flutter_test_config.dart` sets it
for every test file. `push_unconfigured_build_test` sets it to `''`, so
it runs in the main suite instead of being skipped there and run
separately. CI, the Justfile, `scripts/flake-check.mjs` and the
AGENTS/README check commands drop the define, so CI, `just mobile-test`,
pre-push and a bare `flutter test` run the same thing.

I chose this over documenting a canonical command because the define
can't be made automatic: `flutter test` has no config file for defines,
so any doc fix still leaves the obvious command red. A test hook makes
every invocation agree.

The stale test is replaced by one in the push-less file that binds the
production read: with the override cleared, `Env.pushGatewayUrl` must
equal the compiled `BUZZ_PUSH_GATEWAY_URL`. Now that CI compiles without
the define, it fails if the app regains a built-in gateway default,
which the old test never could. The toggle test's mock answers
`takePendingNotificationResponse` with no pending response.

What gets better: one test mode everywhere, and the push-less tests run
in every run. Nothing gets worse: production reads the same compiled
value (the override is never set outside tests, and an `aot-snapshot`
probe shows AOT still folds the getter to the compiled constant and
drops push-only code when the define is absent), and every test still
runs in the push-capable mode CI used before.

### Validation

- On a fresh origin/main worktree with the Hermit toolchain, a bare
`flutter test` failed exactly these 11; the same files passed on the
same machine with the define.
- After the fix, the pre-push `just mobile-check` and `just mobile-test`
(now a bare `flutter test` plus the recipe and secp256k1 checks) pass,
with the push-less tests running instead of skipped.
`scripts/flake-check.test.mjs` passes.
- Falsifiability: re-adding a `defaultValue` to the compiled gateway
fails the new no-default test, and removing `flutter_test_config.dart`
brings back the other 10 failures.
- Unrelated: an earlier full run also failed `community_provider_test`
"discard orphan snapshots and image caches", which waits 500 ms for a
background directory delete. The Mac was running other full suites at
the time; the file passed 8 of 8 runs alone, with and without the
define. It is not touched here.

Signed-off-by: Tom Brow <tomb@squareup.com>
Co-authored-by: Tom Brow <tomb@squareup.com>
Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>

scripts/flake-check.mjs

@@ -91,9 +91,6 @@ export const nodeTestArgs = (file) =>
         "--test",
       ]
     : ["--import", "./test-loader.mjs", "--experimental-strip-types", "--test"];
-// Mirrors the mobile test shards in .github/workflows/_ci-clients.yml.
-export const dartDefine =
-  "--dart-define=BUZZ_PUSH_GATEWAY_URL=https://push.example";
 // Mirrors the postgres-ci default-filter in .config/nextest.toml
 // (`test(/postgres_tests::/)`, `binary(/^postgres_/)` and
 // `test(/(^|::)external_infra[^:]*::/)`): those tests need Postgres, Redis,
@@ -1163,7 +1160,6 @@ function repeatDart(files, iterations) {
       "flutter",
       [
         "test",
-        dartDefine,
         `--file-reporter=json:${reportFile}`,
         ...files.map((file) => file.replace(/^mobile\//, "")),
       ],

scripts/flake-check.test.mjs

@@ -17,7 +17,6 @@ import {
   chooseRust,
   classify,
   cratePackage,
-  dartDefine,
   hunkRanges,
   ITERATIONS,
   infraTest,
@@ -1028,16 +1027,11 @@ test("reports give per-test statuses per iteration", () => {
 });
 
 test("repeat invocations mirror the lanes they stand in for", () => {
-  // node:test flags come from desktop/package.json "test"; the Flutter define
-  // from the mobile test shards; nextest's flake profile must keep retries off.
+  // node:test flags come from desktop/package.json "test"; nextest's flake
+  // profile must keep retries off.
   const script = JSON.parse(read("../desktop/package.json")).scripts.test;
   for (const file of ["x.test.mjs", "x.jsdom-test.mjs"])
     assert.ok(script.includes(`node ${nodeTestArgs(file).join(" ")} "`), file);
-  assert.ok(
-    read("../.github/workflows/_ci-clients.yml").includes(
-      `scripts/mobile-test-shard.sh \${{ matrix.shard }} 3 ${dartDefine}`,
-    ),
-  );
   // nextest reads its config from the workspace root, so each Rust workspace
   // carries the flake profile.
   for (const config of [