Deeply-nested-body test fails on macOS: rejected as INVALID_REQUEST (-32600) instead of PARSE_ERROR (-32700)

Running the test suite on macOS (observed on GitHub macos-latest, Python 3.14) fails one test that passes on Ubuntu and Windows:

FAILED tests/server/test_streamable_http_modern.py::test_modern_post_with_deeply_nested_body_is_parse_error_not_a_crash
>       assert response.json()["error"]["code"] == PARSE_ERROR
E       assert -32600 == -32700

https://github.com/modelcontextprotocol/python-sdk/blob/3a6f2996cdd8358957479791e8b26198c07d6a75/tests/server/test_streamable_http_modern.py#L1011

The response status is 400 in both cases; only the JSON-RPC error code differs. The test expects the RecursionError raised while parsing the deeply nested body to classify as a parse error, but on Darwin it surfaces as INVALID_REQUEST, so the recursion limit is evidently being hit in a different layer there (macOS threads run with a smaller default stack, which moves where the recursion guard strikes).

This is unrelated to any open PR: it fails identically on current main (3a6f299) and on feature branches, reproduced across repeated runs on macos-latest. Repro:

uv sync --frozen
uv run --frozen pytest tests/server/test_streamable_http_modern.py::test_modern_post_with_deeply_nested_body_is_parse_error_not_a_crash

CI currently runs Ubuntu and Windows only, which is why this has gone unnoticed; worth deciding separately whether macos-latest should join the matrix.

_{AI Disclaimer}

Start with tests/server/test_streamable_http_modern.py::test_modern_post_with_deeply_nested_body_is_parse_error_not_a_crash and run the provided uv pytest command on macOS and another platform. Trace where the RecursionError is classified and compare the resulting JSON-RPC codes. Done means the platform-specific behavior is understood and the test's expected parse-error handling is consistent without a crash.