Unit Tests: Add tests for iis7_rewrite_rule_exists()

[pbearne]

The function iis7_rewrite_rule_exists() in src/wp-admin/includes/misc.php lacks unit tests. This ticket covers adding tests to ensure it correctly detects the presence of WordPress rewrite rules within an IIS web.config file using XPath.

…s/misc.php
This PR adds comprehensive unit tests for the iis7_rewrite_rule_exists() function in src/wp-admin/includes/misc.php.

The tests cover:

• Detection of rewrite rules with names "wordpress" and "WordPress".
• Detection of rules with names starting with "wordpress".
• Correct handling of files where the rule does not exist.
• Handling of empty XML configuration and invalid XML content.
• Handling of non-existent files (returns false).

Trac ticket: https://core.trac.wordpress.org/ticket/65148

## Use of AI Tools
AI assistance: Yes
Tool(s): Junie (JetBrains)
Model(s): gemini-3-flash-preview
Used for: Test structure generation, dataProvider implementation, and local environment verification. All tests were verified against WordPress core standards and run in the local Docker environment.

[desrosj]

Adding the tests focus, which is used to indicate a ticket is solely focused on adding tests.