fix: tighten JSON, JSONL, and SSE content type matching

[fallintoplace]

Summary

• require JSON and JSONL content types to match an exact, case-insensitive media type boundary
• keep JSON and JSONL dispatch disjoint, including mixed-case application/jsonL
• preserve supported x-ndjson, x-ldjson, jsonl, and x-jsonl media types with optional parameters and horizontal whitespace
• apply the same case-insensitive boundary handling to Server-Sent Events
• add matcher and end-to-end encode/decode regressions for valid, malformed, suffixed, and unrelated content types

Why

encode_content and decode_content select JSON, JSONL, and SSE behavior by content type. The previous JSONL regex matched unrelated values, while the JSON matcher could claim mixed-case JSONL before JSONL dispatch. Prefix-only matching also accepted invalid suffixes such as application/json-seq and text/event-streaming.

These cases could parse the wrong format, raise parser errors, or return the wrong decoded shape.

Fixes #276.

Test plan

• mise exec ruby@4.0.6 -- ./scripts/test
• mise exec ruby@4.0.6 -- bundle exec rake lint
• mise exec ruby@4.0.6 -- bundle exec rake build:gem
• GitHub Actions: 11 passed, 0 failed