fix: send SEP-2243 streamable HTTP headers

[he-yufeng]

Summary

• add SEP-2243 mcp-method headers to streamable HTTP POST requests
• add mcp-name for tool calls, prompt gets, and resource read/subscribe/unsubscribe requests
• cover the method/name mapping without changing existing session or protocol-version headers

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
• git diff --check

[he-yufeng]

Rebased this branch on current main and force-pushed 3fdd4b3.

The rebase conflict was in tests/shared/test_streamable_http.py after upstream converted the protocol-version checks to the in-process async client fixture. I kept the upstream async protocol-version coverage and re-applied this PR's SEP-2243 mcp-method / mcp-name header regression test on top.

Validated locally:

• uv run --frozen pytest tests/shared/test_streamable_http.py::test_client_includes_protocol_version_header_after_init tests/shared/test_streamable_http.py::test_streamable_http_client_adds_sep_2243_headers tests/shared/test_streamable_http.py::test_server_validates_protocol_version_header tests/shared/test_streamable_http.py::test_server_backwards_compatibility_no_protocol_version -q (11 passed)
• uv run --frozen ruff check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• uv run --frozen ruff format --check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• uv run --frozen pyright src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py (0 errors)
• python -m py_compile src\mcp\client\streamable_http.py tests\shared\test_streamable_http.py
• git diff --check --cached before completing the rebase

[he-yufeng]

Closing this — main now sends the SEP-2243 routing headers directly: _per_message_headers() adds Mcp-Method (and Mcp-Name for tools/call) on modern protocol versions, and _prepare_headers() already pins MCP-Protocol-Version on every request. That covers what this PR added, and the in-tree version goes further (e.g. encoding the tool name and leaving room for the prompts/get / resources/read name params). No reason to keep a competing implementation open. Thanks!