Remove davexmlrpc dependency and resolve security vulnerabilities

- Create custom XML-RPC builder utility in test/xmlrpc-builder.js using existing xmlbuilder dependency
- Replace davexmlrpc usage in test/ping.js and test/please-notify.js with custom builder
- Remove davexmlrpc from package.json dependencies
- Resolves 5 moderate security vulnerabilities (SSRF and prototype pollution)
- Removes 38 vulnerable packages from dependency tree
- All tests pass with zero security vulnerabilities

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: andrewshell