feat: expose variableNames to retrieve all parameters from UriTemplate

[li-yechao]

Added support for retrieving all parameter names from a UriTemplate.

Motivation and Context

Currently, there is no built-in way to extract all parameter names from a UriTemplate. This feature enables developers to programmatically access all variable names within a template, making it easier to handle dynamic URL generation and validation.

How Has This Been Tested?

• Added unit tests to verify that variableNames correctly extracts parameter names from various URI templates.
• Tested in a real application scenario to ensure compatibility with existing URI parsing logic.

Breaking Changes

No breaking changes. This is a non-breaking enhancement.

Types of changes

• New feature (non-breaking change which adds functionality)

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

This feature is particularly useful for scenarios where developers need to inspect or manipulate URI templates dynamically.