feat(realtime): add explicit REST API call for broadcast

[grdsdev]

🔍 Description

This PR ports the feature from supabase-js#1751 to add explicit REST API support for broadcast messages in the Python realtime library.

What changed?

• New method http_send(): Added a new public method to AsyncRealtimeChannel that explicitly uses the REST API for broadcast messages, regardless of WebSocket connection state
• Deprecation warning: Added a warning log when send_broadcast() is called while not connected, alerting users about the implicit behavior
• New dependency: Added httpx[http2] to enable HTTP requests for the REST API calls
• Comprehensive tests: Added 12 new test cases covering all aspects of the http_send() functionality

Why was this change needed?

The current approach can lead users to use broadcast without understanding the underlying mechanism. By providing an explicit http_send() method, users have more control over how their messages are delivered:

• Explicit REST delivery: Users can guarantee REST API usage when they need it
• Better migration path: Helps users gradually migrate from implicit REST fallback to explicit REST calls
• Improved visibility: The deprecation warning makes users aware when they're using REST vs WebSocket

🔄 Breaking changes

• ⚠️ New deprecation warning logged when using send_broadcast() while not connected
  • This is logged as a warning and doesn't break existing functionality
  • Users are encouraged to migrate to http_send() for explicit REST delivery

📋 Checklist

• I have read the Contributing Guidelines
• My PR title follows the conventional commit format: <type>(<scope>): <description>
• I have added tests for new functionality
• All tests pass locally
• Code follows existing patterns and conventions

🧪 Test plan

The PR includes comprehensive test coverage:

• Test http_send() with and without access tokens
• Test payload validation (rejects when payload is None)
• Test timeout handling
• Test error handling for non-202 status codes
• Test error message extraction from response body
• Test custom timeout configuration
• Test private channel support
• Test correct payload structure
• Test deprecation warning in send_broadcast()

All tests pass: pytest tests/test_http_send.py -v

📚 Related

• Source PR: feat(realtime): realtime explicit REST call supabase-js#1751
• Ported to Python with similar API and behavior

🤖 Generated with Claude Code