fix(browser): restore setupWorker lifecycle

[felmonon]

Follow-up for #2650.

Summary

• reuse the browser HTTP and WebSocket sources across worker.start() / worker.stop() cycles
• restore the active ServiceWorkerRegistration return value on redundant worker.start() calls
• stop service-worker keepalive traffic on worker.stop() while preserving post-stop passthrough and restart behavior
• add browser regressions for redundant worker.start(), restart after stop, and keepalive cleanup

Testing

• pnpm build
• pnpm test:browser -- test/browser/msw-api/setup-worker/start/start.test.ts test/browser/msw-api/setup-worker/stop.test.ts test/browser/msw-api/setup-worker/stop/keepalive.test.ts

[kettanaito]

Hi! I am super confused what you are doing here. Are there any problems you were trying to solve with these?

Also, are you using LLMs to generate your pull requests?

[felmonon]

Thanks for calling that out. You're right that I should have made the problem statement much clearer.

This PR was meant to address three browser lifecycle regressions I hit on fix/define-network, relative to the current setupWorker behavior:

• worker.stop(); worker.start() did not re-enable mocking.
• redundant worker.start() no longer returned the active ServiceWorkerRegistration.
• the service-worker keepalive interval continued after worker.stop().

The added browser tests were meant to lock those behaviors down.

I do use LLM tooling sometimes to help draft and iterate, but I only push changes I can reproduce and validate locally. If these behaviors are intentional on this branch, or if I'm missing the intended direction, I'm happy to close this rather than add noise.