Add heartbeat monitoring and auto-reconnect

[gijzelaerr]

Summary

• Adds configurable heartbeat probing via background daemon thread
• Adds automatic reconnection with exponential backoff and jitter
• Supports on_disconnect and on_reconnect callbacks
• Thread-safe reconnection with locking
• is_alive property for connection health monitoring
• Fully backward compatible — all new parameters are keyword-only with defaults that preserve existing behavior

Closes #626, closes #627

Test plan

• 20 tests pass (tests/test_reconnect.py)
• Tests cover: defaults, reconnect on read/write failure, callbacks, max retries exhausted, heartbeat start/stop, alive/dead detection, backward compatibility
• mypy strict mode passes
• ruff check/format passes

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com