fix(fetch): body.blob() sets Blob.type from Content-Type header

[godronus]

Summary

Fixes #311.

When calling body.blob() on a Request or Response, the returned Blob was always created with an empty type, regardless of the Content-Type header on the body. This was uncovered by the WPT pass improvements in #312.

Per the Fetch spec 5.2:

▎ If type is non-null, then set object's type to the result of parse a MIME type with type's value.

This fix reads the Content-Type header from the owning request/response when constructing the Blob in parse_body, extracts the MIME type using the existing extract_mime_type helper, and passes the normalized type through Blob::normalize_type before creating the Blob object.

Changes

• builtins/web/blob.h / blob.cpp: Promotes normalize_type from a file-local free function to a public static method on Blob, so request-response.cpp can call it without duplication.
• builtins/web/fetch/request-response.cpp: In parse_body, after constructing the body buffer, looks up the Content-Type header on the owning object, parses the MIME type, normalizes it, and passes it when creating the Blob.
• tests/wpt-harness/expectations/fetch/api/body/formdata.any.js.json: Updates one previously-failing WPT test to PASS (Consume multipart/form-data headers case-insensitively) — the test was failing because it asserted on the Blob.type value derived from the Content-Type header.

Testing

The fix is covered by the WPT test suite. The Consume multipart/form-data headers case-insensitively test in fetch/api/body/formdata.any.js now passes.

Running WPT test suite after merging Response.clone (#312) creates a new fetch/api/body/mime-type.any.js expectations file. (20 PASS, 0 FAIL)