feat: Adds optional _meta to request input forms.

[mbleigh]

Motivation and Context

The inspector currently has no mechanism of sending _meta information which means it can't be used to inspect/debug custom metadata uses.

How Has This Been Tested?

I've tested this out with my custom MCP server that depends on certain _meta keys and it seems to work well.

Example screenshots:

Breaking Changes

No breaking changes.

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

modelcontextprotocol/modelcontextprotocol#710