feat: Add Phase 1 and 2 improvements for Claude provider

Phase 1.2 - Automatic retry with exponential backoff:
- Add _request_with_retry() method for non-streaming requests
- Retries on 429 (with x-should-retry header), 529, 503 errors
- Exponential backoff with jitter (1s, 2s, 4s max 30s)
- Handles timeouts and HTTP errors gracefully

Phase 1.3 - Streaming idle watchdog:
- Add 90s idle timeout detection (matches vendors/claude)
- Tracks last_event_time and raises TimeoutError on idle
- Prevents indefinite hangs on dropped connections

Phase 2.3 - Cache token tracking:
- Add cache_stats dict to track cache hits/misses
- Track cache_tokens_read and cache_tokens_created
- Add get_cache_stats() method for analytics
- Updates stats during streaming message_delta events

Also includes:
- Temperature fix (skip 0.0 when thinking beta active)
- Rate limit config update (5s default for Claude)