fix(anr): Detect main thread from tombstone via pid matching thread id (#5742)

* fix(anr): Detect main thread from tombstone via pid matching thread id

On Linux/Android the main thread's kernel thread id always equals the process
id, so match a tombstone thread whose id equals the pid as the main thread and
normalize its name back to "main". This is more robust than relying on the
OS-assigned thread name, which some versions/OEMs replace with the process name.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* changelog

* Clarify main thread detection comments

* Fix typo in main thread detection comment

* Remove redundant comments in tombstone parser test

* Fix failing tests

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>