feat: implement server-side streaming for A2A messages and update client to handle SSE responses

[nan-yu]

Description

non-streaming client with streaming agent

custom-components-example.non-streaming.webm

streaming client with streaming agent

custom-components-example.streaming.webm

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.

If you need help, consider asking for advice on the discussion board.

[gemini-code-assist]

Code Review

This pull request introduces streaming support for the custom components example, enabling incremental UI updates. Key changes include the integration of A2uiStreamParser in the Python agent, the implementation of SSE (Server-Sent Events) parsing in the TypeScript client, and middleware updates to handle text/event-stream responses. The UI now features a rendering indicator and processes message chunks as they arrive. Review feedback highlights a potential memory leak in the session parser cache, the need for more robust SSE line parsing, opportunities to reduce code duplication in message extraction logic, and improved error handling for active streams.