fix: validate the event stream MIME type

[bensynapse]

I run Live Tennis API.

The current prefix check rejects Text/Event-Stream and accepts text/event-streaming. This change compares the parsed MIME essence with text/event-stream, ignoring case and parameters.

Combined headers follow Fetch's MIME extraction.
The last valid MIME type wins. Invalid values and */* are ignored. Commas inside quoted parameters are preserved.

The shared HTTP suite adds 39 cases covering case, parameters, invalid subtypes, quoted commas and repeated headers. Sixteen of these fail against the original implementation. A patch changeset is included.

Validation:

• Node 22, 24 and 26, Bun and Deno test suites.
• Chromium, Firefox and WebKit browser suites.
• happy-dom and workerd suites, including their existing expected failures.
• Type compatibility, lint, formatting, build and package checks.

iOS tests were not run on this Linux host.