feat(python): Add request cancellation support

[zhexuany]

Feature Request

Add ability to cancel long-running HTTP requests.

Motivation

Long-running requests cannot be cancelled, which can block application shutdown or cause delays when the recorder is unresponsive.

Proposed Solution

• Use threading.Event for sync client cancellation
• Use asyncio.CancelledError for async client cancellation
• Add cancel() method to request contexts

Files to Modify

• python/axon_client/axon_client/client.py
• python/axon_client/axon_client/async_client.py

Priority

Low - Timeouts provide basic protection