Update URL validation to allow file and other nonstandard schemas

[hmwildermuth]

Update URL validation to allow file and other nonstandard schemas

Motivation and Context

See #86 - this was breaking some servers including the example simple-resource server.

How Has This Been Tested?

Simple resource server now working locally, both ListResources and ReadResources

Breaking Changes

No

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[jspahrsummers]

This is great, thanks!