Fix compatibility false positive on function_exists guarded calls

[shameemreza]

What?

Closes #1363

WP_Functions_Compatibility_Check no longer reports a function call as incompatible with the plugin's minimum WordPress version when that function is guarded by a function_exists() check in the same file.

Why?

The check finds calls by tokenizing the file with token_get_all() and matching any T_STRING followed by (. It has no control flow analysis, so it cannot see that a call is protected by a function_exists() guard. That produces a false positive for a documented backward compatibility pattern: the guarded call never runs on WordPress versions that lack the function.

The reported example guards in __construct() and calls in a separate add() method wired through add_action(), so the guard and the call live in different methods. A narrow "guard wraps the call" rule would not catch that case, which is why this works at the file level.

How?

A new pass collects every function name guarded by a function_exists( 'name' ) string literal in the file, then the call scan skips any call whose name is in that set. Only string literal arguments are trusted, since a dynamic argument cannot be resolved to a name through tokenization. Matching is case-insensitive and tolerates a leading namespace separator.

The trade off is a rare false negative if a file guards a function in one place but also calls it unconditionally elsewhere. That is the safe direction for a linter, since a false positive pushes people to disable the whole check. For reference, PHPCompatibility's equivalent sniff does not honor function_exists() guards either, so this is more forgiving than that baseline.

Testing Instructions

1. Create a plugin with Requires at least: 4.9.6 in its header.
2. Add a class that guards a newer function and calls it from a hooked method:

class Guarded_Feature {
	public function __construct() {
		if ( function_exists( 'register_block_type' ) ) {
			add_action( 'init', array( \$this, 'run' ) );
		}
	}
	public function run() {
		register_block_type( 'my-plugin/gallery', array() );
	}
}

3. Run wp plugin check your-plugin --checks=wp_functions_compatibility.
4. Expected: no errors. Previously: wp_function_not_compatible_with_requires_wp reported on the register_block_type() line.
5. Remove the function_exists() guard and run the check again. Expected: the error returns, confirming unguarded calls are still flagged.

The PR adds a test-plugin-wp-functions-compatibility-with-function-exists-guard fixture and a test_run_with_function_exists_guard_without_errors case covering the guarded pattern.

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Changelog entry

Fix - False positive in WordPress function compatibility check when the call is guarded by function_exists()

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Co-authored-by: shameemreza <shameemreza@git.wordpress.org>
Co-authored-by: davidperezgar <davidperez@git.wordpress.org>
Co-authored-by: ernilambar <nilambar@git.wordpress.org>
Co-authored-by: marekdedic <marekdedic@git.wordpress.org>

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