chore: Implement ClearcutSender HTTP transport for telemetry disabled by default

[ergunsh]

This PR completes the telemetry system by implementing the transport layer for ClearcutSender. It enables actual HTTP communication with the Clearcut backend, handling event batching, rate limiting, and reliable delivery, including robust shutdown handling.

Key Changes:

• HTTP Transport: Implemented fetch-based transport sending POST requests to the Clearcut HTTP server.
• Event Batching: Events are now buffered and flushed periodically (default: 15 minutes) or on shutdown.
• Reliability & Rate Limiting:
  • Server-Side Backoff: Respects next_request_wait_millis from server responses to handle rate limiting dynamically.
  • Transient Error Retries: Failed requests (5xx, 429) result in events being requeued for the next flush.
  • Request Timeouts: Enforced 30s timeout on requests to prevent hanging processes.
  • Session Rotation: Automatically rotates session IDs every 24 hours.
• Safety & Stability:
  • Buffer Overflow Protection: Caps the buffer at 1000 events to prevent memory leaks, dropping oldest events if necessary.
  • Optimistic Removal: Prevents race conditions and duplicate events during shutdown by optimistically removing events from the buffer before sending.
• Testing Improvements:
  • E2E Robustness: Updated E2E tests to use a mock web server instead of relying on the logger to log specific lines.

Implementation Roadmap:
These changes finalize the planned telemetry architecture:

1. CLI & Opt-out Mechanism (Merged)
2. Logger Scaffolding & Integration (Merged)
3. Persistence Layer (Merged)
4. Watchdog Process Architecture (Merged)
5. Transport, Batching & Retries (This PR):
   • Finalized ClearcutSender with HTTP transport, batching, and server-directed backoff strategies.