fix: honor stop() called while start() is still connecting

[RaphaelFakhri]

Summary

stop() did nothing when it ran before the Daily call object existed. start() kept going, joined the call, and emitted call-start after the caller had already ended the session.

This change adds a start generation counter. stop() increments it, and start() checks it after each await that stop() can overtake:

• After the web call is created.
• After the mobile permission wait.
• After join() resolves or rejects.

A cancelled start() returns null without joining, without emitting call-start-failed or call-start-success, and without touching a call object that a newer start() created.

Fixes #26

Testing

Added __tests__/stopWhileConnecting.test.ts with five cases: stop during web call creation, no failure event for a cancelled start, stop then a new start with the first still pending, stop during join(), and stop during the mobile permission wait.

• Without the change to vapi.ts: 5 failed, 0 passed.
• With the change: 5 passed. Full suite: 66 passed, 3 suites.

npx jest