fix: acquire getUserMedia early in start() to prevent mobile gesture timeout

[vapi-tasker]

Summary\n\nFixes the NotAllowedError on mobile browsers (iOS Safari, Chrome on Android) when calling vapi.start(). Mobile browsers enforce a strict "user gesture" policy that only allows getUserMedia() within ~1-5 seconds of a user tap. Previously, the SDK made a network API call to create the web call before invoking getUserMedia() (via DailyIframe.createCallObject), causing the gesture window to expire.\n\nRoot cause: In the start() method, callControllerCreateWebCall (network request) ran first, and DailyIframe.createCallObject({ audioSource: true }) ran second. On slow networks or under load, the API call consumed the gesture window, and Daily's internal getUserMedia() call was rejected by the browser.\n\nFix: Call navigator.mediaDevices.getUserMedia({ audio: true }) immediately when start() is invoked (within the gesture window), then pass the pre-acquired MediaStreamTrack to DailyIframe.createCallObject({ audioSource: track }) so Daily does not call getUserMedia() again.\n\n### Changes\n\n- Early media acquisition: getUserMedia() is called at the top of start(), before any network requests\n- Pre-acquired track passthrough: The track is passed as audioSource to DailyIframe.createCallObject()\n- Graceful fallback: If early getUserMedia() fails, the SDK falls back to the original behavior (Daily handles it)\n- Track cleanup on error: Pre-acquired tracks are stopped if the call creation fails, freeing the microphone\n- User-provided MediaStream support: New mediaStream option in StartCallOptions allows callers to pass their own pre-acquired MediaStream\n- Constructor track detection: If audioSource in constructor options is already a MediaStreamTrack, getUserMedia() is skipped\n\n## Test Plan\n\n- [x] 7 new tests in __tests__/vapi-early-media.test.ts covering:\n - getUserMedia is called before the API call (ordering verified via call tracking)\n - Pre-acquired audio track is passed to DailyIframe.createCallObject\n - Fallback works when getUserMedia fails (Permission denied)\n - Only audio is requested (not video)\n - Pre-acquired tracks are stopped on cleanup when call creation fails\n - User-provided MediaStream is accepted and used (skipping internal getUserMedia)\n - Constructor-provided MediaStreamTrack skips getUserMedia\n- [x] All 13 tests pass (6 existing + 7 new)\n- [x] TypeScript build passes (pre-existing type errors unrelated to this change)\n\n## Linear Issue\n\nResolves VAP-12773