fix(react-native): skip duplicate native report for fatal unhandled JS errors on iOS

[KishanPRao]

Summary

On iOS a fatal unhandled JS error was reported twice (a JS "Unhandled exception" plus a redundant native "Crash"); this drops the native duplicate while keeping the JS report.
Adds a jest test for the marker, and makes the example's "Throw an unhandled error" button a real fatal.

Changes

• UnhandledExceptionHandler.ts: on a fatal error, call CrashReporter.markFatalError() before chaining the previous global handler.
• CrashReporter.ts: add an optional-chained static markFatalError(), a no-op where the native binding is absent (Android).
• BacktraceReactNative.mm: export markFatalError as a blocking-synchronous method so the flag is set before RCTFatal.
• BacktraceCrashReporter.h: declare markJsFatalError.
• BacktraceCrashReporter.mm: stash a jsFatal flag in the crash's PLCrashReporter customData, purge that pending report on next launch, self-clear the flag after 2s.
• unhandledExceptionHandlerTests.spec.ts: cover markFatalError firing on a fatal error and not on a non-fatal one.
• actions.ts: make "Throw an unhandled error" a real fatal via a deferred throw instead of an async rejection.

Note

The 2s self-clear calls saveReportData on the main queue, which can race the module-thread attribute setters; harmless (a stale attribute at worst), can be serialized in a follow-up.

ref: BT-7403

[melekr]

🚀