65183 misc set screen options

[pbearne]

Description:
This PR adds unit tests for the set_screen_options() function in wp-admin/includes/misc.php. These tests ensure that the function correctly saves screen options (like items per page) for the current user, handles non-standard options via filters, and correctly validates input ranges.

The tests cover:

• Saving standard _per_page options.
• Saving custom options using the set-screen-option filter.
• Handling layout_columns options.
• Validating that values are within the allowed range (1-999).
• Correct handling of missing or invalid $_POST data.
• Ensuring the function correctly performs nonce and referer checks.

Trac ticket: https://core.trac.wordpress.org/ticket/65183

AI Disclosure:

• AI assistance: Yes
• Tool(s): Junie (JetBrains)
• Model(s): gemini-3-flash-preview
• Used for: Code analysis, test implementation, and workflow management.

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