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 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 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.