fix(mime): specify charset parameter per MIME type instead of mechanical detection

[renatograsso10]

Closes #4510

Summary

Move the ; charset=utf-8 suffix from a runtime check in getMimeType into the _baseMimes data table itself, and extend it to image/svg+xml, application/xhtml+xml, and application/xml as the issue requested.

Why

Per #4510:

1. Other entries besides text/* benefit from a charset parameter (XML-based formats).
2. Per IANA media types, charset is not universal across all text/* formats — keeping it as data lets us be precise per entry rather than running a generic startsWith("text") check.

The output of getMimeType() for previously-charseted text types is unchanged (still text/<sub>; charset=utf-8), so this is backward-compatible at the call site.

Changes

src/utils/mime.ts

• _baseMimes: append ; charset=utf-8 to css, csv, htm, html, ics, js, mjs, txt, svg, xhtml, xml.
• getMimeType: drop the if (mimeType.startsWith("text")) block; data now carries the charset.
• getExtension: compare the base type (split on ;) so callers can pass either "text/html" or "text/html; charset=utf-8" and still get the right extension. No behavior change for callers passing the raw type.

src/utils/mime.test.ts

• 2 new it blocks: charset values for the new XML-based formats and the additional text types; getExtension round-trip with and without parameters.

Test plan

• npx vitest run src/utils/mime.test.ts — 5/5 pass (3 existing + 2 new)
• npx vitest run src/middleware/compress src/middleware/jsx-renderer src/middleware/serve-static — 56/56 pass (covers the consumers I could trace: getMimeType is used by serve-static, BaseMime is referenced by context.ts)
• npx tsc --noEmit clean

The pre-existing stream/streamSSE onAbort and a few other failures on main are unrelated to this change (verified via git stash + re-run).