fix: deflake watchFile and invoke backspace tests

[tejaskash]

Closes #2140
Closes #2176

watchFile.test.ts (#2140)

Bun registers the macOS kqueue watch off the main thread. Edits written synchronously right after watch() are sometimes never reported (5 of 60 runs locally with no delay, 0 of 60 with a 20 ms settle). The test now settles 100 ms before writing and waits for the callback instead of sleeping a fixed 250 ms. 100 of 100 local runs pass; the previous version failed about 1 in 8 locally.

Test-only: project dev starts watching long before the user edits, so production is unaffected.

invoke.screen.test.tsx (#2176)

The chat frame renders session: <crypto.randomUUID()>. The backspace test asserted that "abcd" disappears from the whole frame, which can never happen when the random UUID contains that hex substring (about 1 in 2500 runs). Reproduced deterministically by seeding a session id containing abcd via the /:sessionId route. The test now types non-hex letters.

Verification

• bun test full suite with --coverage passes locally
• lint and typecheck clean

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[agentcore-devx-automation]

AgentCore Harness Review

Verdict: Looks good

Both flake fixes are well-reasoned and test-only:

• src/io/watchFile.test.ts: The 100 ms settle before writing addresses Bun's off-thread kqueue registration on macOS, and swapping the fixed 250 ms sleep for waitFor(() => calls === 1) asserts on outcome rather than timing. Keeping the fixed 250 ms sleep after controller.abort() is correct — that assertion is that no additional callback fires, which a polling helper can't express.
• src/handlers/harness/invoke/invoke.screen.test.tsx: Switching the typed characters from abcd (hex) to wxyz (non-hex) is the right fix — crypto.randomUUID() output can only contain [0-9a-f-], so collision with the session id in the rendered frame is now impossible rather than ~1/2500. The comment explains why clearly.

No production code is touched, so telemetry doesn't apply. Nothing else to flag.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.08%. Comparing base (c796be3) to head (0be41d9).
⚠️ Report is 7 commits behind head on refactor.

Additional details and impacted files

@@            Coverage Diff            @@
##           refactor    #2204   +/-   ##
=========================================
  Coverage     97.08%   97.08%           
=========================================
  Files           544      544           
  Lines         37768    37768           
=========================================
  Hits          36666    36666           
  Misses         1102     1102           

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[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[Hweinstock]

is there anyway to poll until this is registered somehow? I worry a fixed wait time could be flaky.