Fix: Surface session_terminated event in CLI and Web UI

[VectorCipher]

Fix: Issue #345
This PR fixes a critical bug where a context compaction failure would emit a session_terminated event that was completely unhandled by consumer layers, resulting in a silent hang/death for both CLI and Web UI users.

This PR implements the missing event handlers across the stack to ensure the agent fails loudly and gracefully.

Changes Made:

1. Python CLI (agent/main.py):
   a) Added an explicit handler for session_terminated to stop the shimmer, print the user-facing error message, and correctly unblock the event loop.
   b) Regression Guard: Replaced the silent # Silently ignore other events fallback with a logger.warning to ensure any future unhandled event types are surfaced in the logs instead of failing silently.
2. Frontend Types (frontend/src/types/events.ts):
   a) Added 'session_terminated' to the EventType union for end-to-end type safety.
3. Frontend Transport (frontend/src/lib/sse-chat-transport.ts & useAgentChat.ts):
   a) Added the session_terminated case to the SSE event switch statement.
   b) Connected the event to the existing onError and onProcessingDone side-channels so the Web UI now correctly drops out of the loading state and displays the terminal error banner to the user.
4. Tests:
   a)Added backend tests (test_session_terminated_dispatch.py) to verify the CLI explicitly handles the event and that unknown events trigger the new warning guard.
5. Testing Instructions:
6. Verified backend tests pass successfully.
7. Verified ruff linting and formatting.
8. Verified frontend TypeScript compiler exhaustiveness checks pass.