fix: send Origin header for streamable HTTP

[he-yufeng]

Fixes #2727.

Summary

• derive an Origin header from the streamable HTTP endpoint for client requests
• keep a caller-provided Origin on custom httpx.AsyncClient headers
• cover the default and custom-Origin paths in streamable HTTP client tests

To verify

• uv run pytest tests/shared/test_streamable_http.py -q
• uv run ruff check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• uv run ruff format --check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• uv run pyright src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• python -m py_compile src\mcp\client\streamable_http.py tests\shared\test_streamable_http.py

[he-yufeng]

Closing this during backlog cleanup. The python-sdk queue has too many open PRs, and this older/no-human-signal item is lower priority than the remaining PRs that have recent validation, review discussion, or clearer current value. I can reopen or refresh it if maintainers want this specific change.