Adds optional _meta parameter support to ClientSession.call_tool and ClientSessionGroup.call_tool methods, enabling users to pass request metadata (such as progress tokens or user context) in a clean, explicit, and backward-compatible way.

Motivation and Context

Currently, users who need to pass metadata (like progress tokens or user context) to tool calls have to work around the SDK's limitations by either:

1. Creating custom session classes that override call_tool
2. Passing metadata through the arguments parameter (which is not semantically correct)

This change provides a proper, first-class way to pass request metadata while maintaining full backward compatibility.

How Has This Been Tested?

• Added comprehensive unit tests for both ClientSession.call_tool and ClientSessionGroup.call_tool with _meta parameter
• Verified that existing functionality remains unchanged (backward compatibility)
• Tested that _meta data is properly passed through to the underlying request structure
• All existing tests continue to pass

Breaking Changes

No breaking changes. The _meta parameter is optional and defaults to None, so existing code will continue to work without modification.

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

• The implementation leverages the existing RequestParams.Meta structure that was already defined in the types
• The _meta parameter is passed as the _meta field in CallToolRequestParams, which inherits from RequestParams
• This change is minimal and focused, affecting only the client-side tool calling interface
• The feature enables advanced use cases like progress tracking, user context passing, and other metadata requirements without requiring users to create custom session classes