Tests: Add PHPUnit tests for wp_json_encode(

[pbearne]

The wp_json_encode() function in wp-includes/functions.php is a wrapper around json_encode() that provides additional logic, such as handling UTF-8 encoding issues and applying the wp_php_error_args filter.

Currently, this function lacks dedicated unit test coverage in the core suite. This patch introduces a new test class Tests_Functions_WpJsonEncode to verify:

• Basic encoding of arrays and objects.
• Correct handling of JSON encoding options.
• Handling of depth parameters.
• Ensuring the function returns the expected JSON string or false on failure.
• (Optional) Verification of UTF-8 sanitization behaviour for invalid strings.

This PR introduces dedicated unit tests for the wp_json_encode() function in wp-includes/functions.php.

The tests cover:
•Standard encoding of diverse data structures.
•Correct application of JSON encoding constants/options.
•Enforcement of maximum recursion depth.
•The function's ability to sanitize and encode strings containing invalid UTF-8 sequences, which is a key feature of the WordPress wrapper over native json_encode().

Trac ticket: https://core.trac.wordpress.org/ticket/65652

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

[wildworks]

Since 7.1 Beta 1 has already been released and these functions were not newly introduced with the 7.1 release, I'd like to remove the milestone.