docs: update code snippet markers to match refactored sample identifiers

[sokoliva]

Fixes: https://a2a-protocol.org/latest/tutorials/python/6-interact-with-server/#understanding-the-client-code

Sample update: #508

[gemini-code-assist]

Code Review

This pull request updates the fragment identifiers for code snippets in the docs/tutorials/python/6-interact-with-server.md file, changing message/send to message_send and message/stream to message_stream. The review comments suggest an improvement to enhance the long-term stability of these documentation links by replacing refs/heads/main with a specific Git commit hash or tag to prevent future breakage if the sample code changes.