Docs: Fix duplicate-word and spelling typos in comments and docblocks

[itzmekhokan]

Corrects duplicated words (such as "the the", "in in", "on on", "is is", "from from", "and and") and a misspelling ("Compatability") in WordPress-owned docblocks and inline comments across src/ and the PHPUnit test suite. These are comment-only edits with no functional or behavioral change.

Third-party and bundled libraries (ID3, PHPMailer, SimplePie, plupload, jQuery UI, etc.) are intentionally left untouched, as they are maintained upstream.

Trac ticket: https://core.trac.wordpress.org/ticket/65384

Use of AI Tools

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