Fix stdio server newline translation on Windows

[yyzquwu]

Summary

• Disable Windows CRLF translation in stdio_server() by setting newline="" on the default stdio wrappers.
• Add a regression test for the default stdout path so JSON-RPC frames stay LF-only.

Fixes #2433.

Verification

Windows 11, Python 3.13.13:

tests/server/test_stdio.py::test_stdio_server PASSED
tests/server/test_stdio.py::test_stdio_server_uses_lf_newlines_with_default_stdout PASSED
tests/server/test_stdio.py::test_stdio_server_invalid_utf8 PASSED
3 passed

b'{"jsonrpc":"2.0","id":3,"method":"ping"}\n'

Branch coverage for tests/server/test_stdio.py: 100.00%.

Codex review passed before commit.

[StantonMatt]

Checked head 86176c5 on Windows with Python 3.13.13.

Local validation passed:

• uv run --frozen pytest tests/server/test_stdio.py -q -> 3 passed
• uv run --frozen ruff check src/mcp/server/stdio.py tests/server/test_stdio.py
• uv run --frozen pyright src/mcp/server/stdio.py tests/server/test_stdio.py

I also ran a default-stdout byte probe through stdio_server() with a JSONRPCRequest(id=99, method="ping"); the emitted frame ended in 0a and did not contain 0d0a:

7b226a736f6e727063223a22322e30222c226964223a39392c226d6574686f64223a2270696e67227d0a

That covers the Windows newline-translation boundary from #2433 for me.