fix(negotiation): honor Accept exclusions and specificity before selecting a representation #10

Problem and contract

Accept::getBestMatch sorts client ranges by q and returns the first matching server type. This selects explicitly unacceptable representations and lets a wildcard override a more specific range. The documented claim of proper HTTP negotiation does not match RFC 9110 sections 12.4.2 and 12.5.1.

Evidence

Audited commit: a429fbe9a7efce6cee50b82e4a981ea7e3b47a5f.
Selection loop, parsing/sort.
Normative HTTP semantics: determine each representation's quality using its most specific matching media range; q=0 means not acceptable.

Regression cases

Run these with mb_trim available (the separate PHP compatibility issue #9 must not mask the negotiation behavior):

Acceptance criteria

• Compute effective q per candidate from the most specific applicable range; exclude q=0 before ranking candidates.
• Match media types and parameter names case-insensitively.
• Add tests for these cases, including wildcard exclusions, not only equal-q positive examples.
• Keep unsupported-type behavior returning null.

Priority P2; high confidence from branch tracing and RFC. Static validation only: no PHP executable available. Existing AcceptTest lacks these cases; all pre-existing issues/PRs were checked (#1–#8), and #9 is a separate runtime compatibility issue. No source or release changes made.