test: isolate flaky service and tab checks - #42582

AI code review — automated review for reference; please use your judgment.

Scope: de-flakes two suites: extracts service registration into services/service-registration.ts (verbatim move) so the stale-registration test exercises it directly instead of spawning a real server subprocess, and makes the TUI tabs test's emit() await actual event delivery through the client listener instead of racing.

• The registration test rewrite is a big win: same assertions (file contents, cleanup removal) with zero process spawning — faster and deterministic. The moved code is character-identical, so production behavior is untouched.
• Coverage note: the old test also implicitly covered env/port wiring of a real serve --service boot (it wrote a config file and asserted the bound port). That integration signal is gone; fine for stability, but consider keeping one slower end-to-end registration test behind a tag so the wiring doesn't rot silently.
• The tabs emit() synchronization is the right idea (resolve only after the event round-trips through the client's event channel), but it has no timeout — if delivery ever breaks, the test hangs until the global runner timeout instead of failing with a clear message. Wrapping the promise in a short Effect.timeout-style guard would fail fast with context.
• Identity check delivered.details.id !== event.id assumes ids survive the bridge intact — true today; noting in case the synthetic-id path changes.

AI code review - automated review for reference; please use your judgment.

• packages/tui/test/context/session-tabs.test.tsx:144-150 - the new emit promise resolves only when the exact event id is observed via client.event.listen; if delivery is dropped/filtered the test hangs indefinitely with no diagnostic. Race it against a timeout (like the old wait) so failures report instead of hanging CI.
• packages/cli/test/service.test.ts:444-474 - good de-flaking by testing ServiceRegistration.register directly; note the old spawn-based test also exercised incumbent replacement during the bind race end-to-end, which is now only covered indirectly - worth keeping an eye on that path in e2e.
• packages/cli/src/services/service-registration.ts - extraction is a faithful move (no behavior change), which keeps this PR purely a test-isolation change - clean.

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