fix(server): match Content-Type case-insensitively in StreamableHTTP

[ly-wang19]

Summary

StreamableHTTPServerTransport._check_content_type compared the request's Content-Type media type case-sensitively, so a spec-valid request with a mixed/upper-case type (e.g. Content-Type: Application/JSON) was rejected with 415 Unsupported Media Type.

Media types are case-insensitive (RFC 9110, §8.3.1), and the sibling _check_accept_headers already lowercases Accept media types before comparing — _check_content_type just didn't.

Fix

Lowercase the parsed Content-Type media type before comparing to CONTENT_TYPE_JSON, consistent with _check_accept_headers.

Test

Added test_check_content_type_is_case_insensitive (the _check_content_type rejection path was previously marked # pragma: no cover). It asserts application/json, Application/JSON, APPLICATION/JSON, and ; charset=utf-8 variants are accepted, and text/plain is still rejected.

Verified red→green: the new test fails on main (mixed-case rejected) and passes with the fix. uv run pytest tests/shared/test_streamable_http.py → 70 passed; ruff check / ruff format --check clean on both files.

[maxisbey]

Thanks for the PR, and sorry it sat here without a proper review.

We're closing most of the open PR backlog. v2 is out and changed a lot of the SDK, so many older PRs no longer apply as written, and we're a small team that realistically doesn't have the capacity to work through the rest.

If this still matters to you on v2, the most useful thing you can do is open an issue (or comment on the existing one) with your use case and a repro. Hearing why it matters to you is what we use to decide what to prioritise.

_{AI Disclaimer}