test(core): poll the installer PTY instead of racing it

[StvLi]

A docs-only PR (#52) went red on check (22), and the same job passed on an immediate re-run. That is the PTY test, and this fixes it.

Evidence

FAIL tests/tools.spec.ts > ros2_install interactive flow (mock installer, no network)
     > start -> send -> status -> stop drives the installer menus via PTY
AssertionError: expected '' to contain '众多工具'

Run	check (22)
PR #52, first run	fail
PR #52, re-run, no code change	pass
PRs #48 / #49 / #50 / #51, both Node versions	pass

Cause

Not the environment — a race in the test. ros2_install drives the installer inside a pty daemon whose output arrives asynchronously, but the test read status exactly once, immediately after start, and required the menu to be present already:

await t.execute({ action: 'start', ... })
const s1 = await t.execute({ action: 'status', session })   // ← no wait at all
expect(s1.data.output).toContain('众多工具')                  // ← relies on the daemon having won

The next two steps were the same hazard written as a guessed duration — setTimeout(800) and setTimeout(2500). Those lower the odds without removing the race, and pay that cost on every single run.

Fix

statusUntil(t, session, predicate, timeoutMs) — a bounded 100 ms poll that returns the moment the awaited output appears. Each step now waits for the specific thing it asserts instead of for a duration, and the final step's predicate also requires state to have reached exited rather than assuming output and state flip together. Timeout 15 s → 30 s so a genuine failure still reports as which assertion never arrived, not a generic timeout.

Verification

pnpm run typecheck passes. The test is it.skipIf(!ptyUsable), and this host cannot allocate a pty, so the PTY path is exercised by CI only — the local-skip / CI-truth split already recorded in §19.8 of the maintenance log. Both check (22) and check (24) are the real verification here.

Worth noting as a category, since this repository has now hit it twice: a flaky gate is a maintenance liability in its own right. The maintain log's own guidance is "push → open a PR → wait for CI green before merging", and that only means something while green is informative.