Build/Test Tools: Add unit test for send_frame_options_header()

[irozum]

Adds PHPUnit coverage for send_frame_options_header() in wp-includes/functions.php, which currently has no test coverage. This change is test-only and adds no production behaviour — it does not touch any file under src/. The new test calls the function directly and asserts, via xdebug_get_headers(), that both the X-Frame-Options: SAMEORIGIN and Content-Security-Policy: frame-ancestors 'self'; headers are sent — the function's entire observable behaviour, since it takes no parameters and returns nothing. The test is marked @requires function xdebug_get_headers and runs in a separate process (@runTestsInSeparateProcesses), following the existing precedent for header assertions in tests/phpunit/tests/oembed/headers.php, so it is skipped rather than failing on environments without the Xdebug extension loaded. I verified the test is meaningful by temporarily changing the X-Frame-Options value sent in send_frame_options_header() and confirming the test failed, then reverted the change and confirmed it passes again. I did not add a test for the headers_sent() guard clause (the early return when headers were already sent) — PHPUnit's beStrictAboutOutputDuringTests buffers test output internally, so producing real output from within a test does not actually flip headers_sent() to true in this harness, and I did not want to force a fragile workaround for that branch.

Trac ticket: https://core.trac.wordpress.org/ticket/59851

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Ticket triage/selection, reading the existing implementation and test conventions, writing the test file, and running the verification loop (green/red/green). All output was reviewed by me before committing.

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