[Test] Shared awaitCondition helper to replace sleep-based waits

[pathosDev]

Rationale

The test suite contains ~684 sleep( call sites, with sleep re-declared per file as Bun.sleep (not portable to the Node/Deno smoke runs). Fixed sleeps before assertions are the primary flake source under load (see #290) and inflate suite duration. A shared polling helper that asserts on observable state instead of elapsed time removes most of them.

Scope

• tests/util/awaitCondition(predicate, { timeoutMs, intervalMs, label }) with diagnostic timeout messages (style of tests/util/AsyncAssertions.ts), plus one portable sleep util for the few legitimate uses.
• Convention documented: assert state, not time; prefer TestProbe.expectMessage / ManualScheduler where applicable.
• Migrate the top flaky suites first (see [Test] Parallel-test flake catalog, stress verification and diagnosis doc #290 list), then ratchet.
• Biome rule (via the Biome adoption issue) banning raw setTimeout/Bun.sleep in tests/ outside the util.

Documentation

• Testing docs note (EN + DE); CONTRIBUTING testing section.

Acceptance

• Converted top-flake suites stable across 10 consecutive full runs.
• Lint prevents new raw-timer usages in tests/.

Relates

Improvement program M1 (foundation). #290, #325.