feature: contain sdk failures instead of crashing the host app

[KishanPRao]

Summary

A failure inside the SDK no longer crashes or changes the host app, the SDK turns that feature off and logs a Backtrace warning.

Changes

• failureLog.ts: new, one-line Backtrace: warning with the error class, never the error message.
• index.ts: exports failureLog.
• AttributeManager.ts: skips a provider that throws.
• BreadcrumbsManager.ts: addBreadcrumb never throws, re-entry guard for interceptors that log.
• BacktraceCoreClient.ts: send() never throws or rejects.
• BacktraceDatabase.ts, BacktraceMetrics.ts: warn instead of leaving an unhandled rejection.
• BacktraceClient.ts: initialize() never throws, falls back to a disabled client, defaults missing app attributes to unknown, guards each init and dispose step, dispose() clears the singleton.
• BacktraceClientBuilder.ts: runs JS-only without the native storage modules.
• CrashReporter.ts: warns on every path that turns native crash reporting off.
• UnhandledExceptionHandler.ts: the app's global handler always runs.
• AnrWatchdogHandler.ts: guards the ANR listener and native calls.
• WebRequestEventSubscriber.ts: keeps synchronous XHR synchronous.
• ErrorBoundary.tsx: warns instead of throwing without a client.
• BacktraceReactNative.java: survives a missing native library, converts attribute values to strings.
• BacktraceDirectoryProvider.java: lists a missing directory as empty.
• BacktraceFileSystemProvider.java: guards unlink and exists.
• StreamWriter.java: thread-safe writer map.
• AnrWatchdog.java, BacktraceAnrWatchdog.java: an exception in the watchdog no longer kills the app.
• Android attribute providers: return partial attributes when a read throws.
• iOS attribute providers, OomWatcher.mm, BacktraceCrashReporter.mm: nil-safe.
• BacktraceReactNative.mm: catches native init and update exceptions.
• failureContainmentTests.spec.ts, selfHealingTests.spec.ts: new.

ref: BT-7704