[build-tools] Handle detached Android emulator exits

[sjchmiela]

Summary

Observe the detached Android emulator process promise immediately after startup so expected exits during retry cleanup do not surface as unhandled promise rejections.

The emulator is managed through adb after launch. When a startup attempt times out and cleanup kills the emulator, the underlying SpawnPromise can reject while the retry loop is handling the readiness failure. Observing the promise keeps Sentry from capturing that cleanup path as an unhandled SIGKILL while preserving the returned promise for callers that still await it later.

Test plan

• yarn run -T oxfmt packages/build-tools/src/utils/AndroidEmulatorUtils.ts
• yarn workspace @expo/build-tools typecheck

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[github-actions]

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File Patterns	Mentions
**/*	@douglowder

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 59.67%. Comparing base (6d02728) to head (df4cbb9).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ages/build-tools/src/utils/AndroidEmulatorUtils.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3998      +/-   ##
==========================================
- Coverage   59.67%   59.67%   -0.00%     
==========================================
  Files         940      940              
  Lines       41260    41261       +1     
  Branches     8666     8666              
==========================================
  Hits        24617    24617              
- Misses      16549    16550       +1     
  Partials       94       94              

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[Copilot]

Pull request overview

This PR prevents expected Android emulator process exits (especially those triggered during retry cleanup) from surfacing as unhandled promise rejections by proactively “observing” the detached emulator SpawnPromise immediately after launch.

Changes:

• Attach a non-blocking asyncResult() observer to the detached emulator SpawnPromise right after unref().
• Add inline comments documenting why the emulator promise is intentionally observed but not awaited at startup.

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