Feeds: Store the resolved MIME type for enclosures

[benitoalba]

do_enclose() resolves each URL's MIME type in $type, but saves $mime from the extension lookup loop. A URL without an extension can raise an undefined-variable warning, an unknown extension can save an unrelated MIME type, and a later URL can inherit the previous URL's type.

Save $type so enclosure metadata uses the type that was already checked. The seven new test cases cover those failures, audio and video headers, query-only URLs, extension precedence, and rejection of non-media content. Regenerating the PHPStan baseline removes the obsolete undefined-variable entry.

This restores the variable used before r20435. @murshed also included this fix in #12721, which closed when an unrelated Trac ticket was resolved. This PR focuses on enclosures and adds regression coverage.

Trac ticket: https://core.trac.wordpress.org/ticket/65817

Testing

Tested in Core's Docker environment on PHP 8.3.33 and PHPUnit 9.6.37.

• Before the fix, the seven new cases produced three undefined-variable errors and two MIME-type assertion failures.
• ./vendor/bin/phpunit tests/phpunit/tests/functions/doEnclose.php: 35 tests, 36 assertions, passed.
• ./vendor/bin/phpunit --group functions,feed: 1,330 tests, 2,930 assertions, no failures or errors. The run reported 27 PHPUnit deprecation warnings about error expectations in unchanged tests.
• Full PHPStan analysis after regenerating the baseline and clearing its cache: passed.
• WordPress PHPCS: no errors; the test file is clean. The source file has one unchanged SQL warning at line 1858.
• PHPCompatibility checks on both PHP files: passed.

Use of AI Tools

AI assistance: Yes
Tool(s): OpenAI Codex
Used for: Issue research, implementation, regression tests, local validation, and this description.

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This pull request is for code review only. Please keep other discussion in the Trac ticket. Do not merge this pull request.

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