fix(server): accept mixed-case JSON Content-Type

[CRYPTONIKAV]

Closes #3670

What I changed

• src/mcp/server/streamable_http.py: lowercase the parsed parts in _check_content_type, mirroring the sibling checks, so mixed-case JSON media types pass the transport check.
• tests/interaction/transports/test_hosting_http.py: interaction test with mixed-case variants (incl. charset param) asserting 200, plus text/plain negative control asserting 400.
• tests/server/test_streamable_http_content_type.py: direct unit tests of _check_content_type (accept + reject paths), covering the transport-level 415 branch.

How it works

Media types are case-insensitive (RFC 9110 section 8.3.1). The middleware already lowercases before comparing, but the transport compared raw parts against lowercase application/json, so Application/JSON passed the middleware and then failed the transport. Lowercasing the parsed parts aligns both layers. Verified: the new tests fail on main (RED) and pass with the fix (12 interaction + 9 unit tests GREEN, ruff check+format clean).

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3670.

If a maintainer assigns you to #3670, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.