Unify async test assertions on a shared helper

[polRk]

What

Standardize the test suite on a single async-assertion style — vitest expect matchers plus one shared promise-settlement helper — replacing scattered hand-rolled .then() idioms. Test-only; no product behavior changes.

Why

The suite mixed several ad-hoc async-assertion idioms: bare .then() flag probes for "still pending", Promise.race([p, setTimeout(...)]) timeouts for liveness, and a per-file re-invented trackCommit. This was inconsistent and tripped two oxlint warnings (promise(always-return), no-unmodified-loop-condition). AGENTS.md already declares vitest expect the standard, but it wasn't enforced for the async cases.

Changes

• Add a shared, dependency-free track(promise) → { settled, rejected, reason? } helper next to each package's settle() in *.fixtures.ts (core, coordination, topic writer + reader), plus a new packages/fsm/src/promise.fixtures.ts.
• Replace hand-rolled pending-probe flags (let f = false; p.then(() => (f = true))) with track() across endpoints, writer, session-runtime, session-registry, and fsm queue.
• Replace Promise.race([p.then(...), setTimeout(...)]) liveness checks with await expect(p).resolves.*, bounded by the test timeout (coordination client, topic reader).
• Swap reader-rebalance's local trackCommit for the shared track().
• Document the one canonical vitest-expect assertion style (rejects / resolves / pending-probe) in AGENTS.md Testing Rules.
• Net effect: clears both oxlint warnings (48 → 46 total); no behavior change.

Not a breaking change. Intentionally left as-is (documented in review, not the target anti-pattern): session-pool dual-outcome captures, the election/mutex integration probes with their justified heuristic sleeps, and the fsm sleep().then('label') race harness.

Testing

• npm run build — passes (fixtures are compiled, so track/Settlement are type-checked).
• npm run test:uni — 1023 passed, 1 skipped.
• reader-rebalance integration test — run live against local YDB, passes.
• npm run lint — exit 0, 46 warnings (down from 48; promise(always-return) and no-unmodified-loop-condition cleared, none added).
• Edited test files type-checked clean.

Checklist

• Changeset added — N/A: test-only change, no published/consumable API surface affected.
• README updated — N/A: no public API change.

Separate from the core-package refactor (#638).