fix: harden subscribeToAgent with try-catch and add core-level tests

Address review findings from PR CopilotKit#3734:

- Wrap flushPending callback invocations in try-catch so a thrown
  exception in onMessagesChanged/onStateChanged does not permanently
  deadlock the throttle state machine or skip the sibling flush
- Add 21 dedicated unit tests for CopilotKitCore.subscribeToAgent
  covering leading/trailing edge, shared window (bidirectional),
  burst coalescing, run lifecycle passthrough, unsubscribe cleanup,
  resolution cascade, invalid values, and exception safety
- Use named CopilotKitCoreSubscription return type
- Fix Angular test stub to accept the options parameter and document
  that it bypasses throttle logic

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mxmzb

packages/angular/src/lib/agent.spec.ts

@@ -83,7 +83,9 @@ class CopilotKitStub {
     runtimeTransport: "auto" as const,
     runtimeConnectionStatus: CopilotKitCoreRuntimeConnectionStatus.Disconnected,
     headers: {} as Record<string, string>,
-    subscribeToAgent: (agent: any, subscriber: any) =>
+    // Passthrough stub — does not implement throttle logic.
+    // Throttle behavior is covered by core-level unit tests.
+    subscribeToAgent: (agent: any, subscriber: any, _options?: any) =>
       agent.subscribe(subscriber),
   };