Validate metadata keys

[samchenatti]

This PR validates types.RequestParams.Meta to enforce the MCP specification for the _meta field.

Motivation and Context

This change ensures that users won't rely on invalid key names when using the _meta field.

How Has This Been Tested?

It was tested in a local client/server setup.

Unit-tests were also added to ensure the correct behavior.

Breaking Changes

There are breaking changes in the case some users are already using invalid _meta keys.

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

It is the result of discussion in #1231

Although I'm not entirely sure we should enforce this validation through the SDK, I'm offering this approach as a possible option.