Display "completions not supported" for resource templates which do not support completions.

[balajmarius]

Motivation and Context

This adds the functionality requested #458 and a small dark screen fix.

How Has This Been Tested?

Added combobox.test.jsx.

Types of changes

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

Checklist

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