BT-6886: fix: label unhandled promise rejections as 'Unhandled rejection'

[GT1990]

Summary

• Adds 'Unhandled rejection' to BacktraceErrorType and uses it on the async error paths in browser, node, and react-native SDKs. Sync paths (window error event, uncaughtException, RN error handler/boundary, Android native, React error boundary) keep 'Unhandled exception'.
• Fixes BT-6886: customer (Netflix Hub / Unity WebGL) was seeing "Failed to fetch" events surface as error.type: "Unhandled exception" even though they were unhandled promise rejections from Unity's WASM bootstrap fetch, not actual synchronous crashes.
• The UnhandledPromiseRejection classifier already existed and was correct; the error.type attribute now matches it. Customers can filter sync crashes from async noise without union queries.

Customer-facing behavior change

Dashboards and alerts filtering on error.type = "Unhandled exception" will see fewer matches after this lands. Async rejections (likely the majority of browser/RN traffic) move to error.type = "Unhandled rejection". Existing reports keep their old labels - no data migration.

If you have hardcoded filters and want a combined view, union the two values.

What changed

File	Change
packages/sdk-core/src/model/report/BacktraceErrorType.ts	added 'Unhandled rejection' to the union
packages/browser/src/BacktraceClient.ts	unhandledrejection listener now uses new label
packages/node/src/BacktraceClient.ts	both uncaughtExceptionMonitor (branched on origin) and dedicated unhandledRejection listener now use new label
packages/react-native/src/handlers/UnhandledExceptionHandler.ts	both Hermes and non-Hermes promise-rejection paths now use new label
packages/*/tests/...unhandledErrorTests.spec.ts (3 new files)	regression tests asserting new label on async and old label on sync

Test plan

• npm run lint clean
• npm test --workspace=@backtrace/sdk-core (320 tests)
• npm test --workspace=@backtrace/browser (59 tests, includes new regression)
• npm test --workspace=@backtrace/node (53 tests, includes new regression)
• npm test --workspace=@backtrace/react-native (36 tests, includes new regression)
• Manual end-to-end against a real Backtrace endpoint (node script, browser HTML): async submitted with error.type: "Unhandled rejection", sync still "Unhandled exception". Two rxids confirmed in the test universe.
• If anyone has a React Native or Electron app already wired up against this SDK and wants to do a quick real-device smoke check before merge, that'd be appreciated. Unit-level coverage is in place but device runs would be additive.