fix(output): match JSON media types case-insensitively

[xu91102]

Normalize the media type inside the JSON formatter before its format-detection checks. Valid uppercase and mixed-case JSON types currently bypass formatting, including Application/Problem+JSON. Response headers and payload content are unchanged.

Two CLI-entry regressions reproduce the original failure; the JSON documentation now states case-insensitive media-type handling.

Validation: 417 tests pass, 3 expected failures, across test_json.py and test_output.py; Flake8 and diff checks pass. The suite's invalid-host test initially failed because the local proxy intercepted the request; it passes with proxy environment variables removed for that test process. Existing resource warnings remain. No full OS/Python matrix was run.

Self-review: limited to the JSON formatter, response regressions and its documentation. Authored with OpenAI Codex assistance.

Fixes #1958.

Remote CI note: the coverage job fails two Big5 charset-detection tests. Both failures also reproduce in an untouched checkout of base 5b604c3 with the same local dependency environment; they are outside this MIME-formatting change.