test(server): replace flaky fs-watch e2e burst test with FsWatcherService unit test

Problem

packages/server/test/fs-watch.e2e.test.ts AC #2 ("burst > 500 changes inside 200ms window → truncated:true") is flaky under concurrent test runs.

The test creates 600 files with writeFileSync in a loop and expects chokidar to deliver more than 500 change events inside one 200ms debounce window. On slow machines or when vitest workers compete for disk IO, the 600 writes span multiple debounce windows, so no single window exceeds 500 events and truncated: true is never emitted. The assertion then fails with a timeout or expected false to be true.

Root cause

The e2e test relies on real file IO speed to simulate a burst. File IO speed is not controllable in a test environment, making the test non-deterministic.

Proposed fix

Move the truncated-burst assertion from the e2e test into a FsWatcherService unit test in packages/server/test/services.test.ts:

• Drive the watcher with a fake FSWatcher that emits 600 synthetic all events.
• Assert that FsWatcherService sets the internal truncated flag and emits an event.fs.changed frame with truncated: true and the correct count after the debounce window expires.

This tests the exact same production logic without depending on real file IO timing.

Scope

• Remove or weaken AC #2 in packages/server/test/fs-watch.e2e.test.ts.
• Add a unit test for the truncated path in packages/server/test/services.test.ts.

I will open a PR shortly.

Start with packages/server/test/fs-watch.e2e.test.ts AC #2 and the FsWatcherService tests in packages/server/test/services.test.ts. Use the existing service test setup and drive a fake FSWatcher with 600 synthetic all events, then verify the debounced event.fs.changed frame has truncated: true and the correct count. Remove or weaken the flaky e2e assertion and run the affected server tests.