feat: Add exception recording.

[kinyoklion]

Adds basic exception recording and updates example app.

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Note

Adds Observe.recordException and integrates Flutter/zone error reporting in the example app with a button to trigger an unhandled error.

• Library (lib/src/observe.dart):
  • Add Observe.recordException to record exceptions (with optional stackTrace and attributes), creating a _launchDarklyErrorSpanName span and ending it.
  • Introduce error span name constant: _launchDarklyErrorSpanName = 'launchdarkly.error'.
• Example App (example/lib/main.dart):
  • Hook FlutterError.onError to Observe.recordException for framework-handled errors.
  • In runZonedGuarded error handler, call Observe.recordException for uncaught errors.
  • Add ElevatedButton to deliberately throw an exception ("Trigger unhandled error").

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