Abilities API: Authentication

[n7studios]

Summary

Adds authentication and instructions for connecting AI clients to the Kit WordPress Plugin MCP.

Testing

• PluginSettingsMCPCest:testGenerateAndRevokeApplicationPassword: Tests that generating and revoking an Application Password works with no errors and that the MCP server is accessible using the Authorization Header generated via the Application Password.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)