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

[coveralls]

Pull Request Test Coverage Report for Build 18695423238

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 72 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.02%) to 93.906%

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Files with Coverage Reduction	New Missed Lines	%
src/postgrest/base_request_builder.py	6	91.15%
src/postgrest/_sync/request_builder.py	11	90.84%
src/postgrest/_async/request_builder.py	11	90.84%
src/realtime/_async/channel.py	44	75.2%

Totals
Change from base Build 18355756900:	-0.02%
Covered Lines:	8829
Relevant Lines:	9402

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💛 - Coveralls