Replace assertEquals with assertSame for stricter checks

[shreyasikhar]

PR Description

• Updates multiple PHPUnit tests to use assertSame() instead of assertEquals() to enforce strict type comparisons.
• Adds an explicit (int) cast for post_author in the bulk edit test to avoid false positives due to string/integer mismatches.
• Touches tests covering admin bulk edit behavior, REST API endpoints (attachments, global styles, wp-abilities), and theme preview hooks—no production/runtime code changes.

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Trac ticket: https://core.trac.wordpress.org/ticket/64895

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Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot
Used for: Find the files where assertEquals() can be replaced with assertSame().

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