Fix false-positive JSONL content type matches

[fallintoplace]

Summary

• require JSONL content types to use a recognized application/ subtype and stop at the subtype boundary
• preserve valid x-ndjson, x-ldjson, jsonl, and x-jsonl matches, including parameters, optional whitespace, and mixed casing
• add regression coverage for unrelated values that previously matched, plus subtype suffixes such as application/jsonlines

Why

decode_content uses this matcher to select line-delimited JSON parsing. The previous regex could therefore send unrelated response bodies through the JSONL decoder and raise parser errors or return the wrong decoded shape.

Fixes #276.

Test plan

• bundle exec rake TEST=test/openai/internal/util_test.rb test
• bundle exec rake lint