SEP-1577 follow-ups

[DaleSeo]

Motivation and Context

The SEP-1577 implementation in PR #628 doesn't fully enforce the spec's MUST requirements.

This PR adds a validate() method to CreateMessageRequestParams that checks role constraints (ToolUse only in assistant messages, ToolResult only in user messages), ensures tool result messages don't mix in other content types, and verifies that every ToolUse is balanced with a corresponding ToolResult. A matching validate() on CreateMessageResult enforces that the role is always "assistant".

How Has This Been Tested?

8 new unit tests covering all validation paths (role constraints, exclusivity, balance, result role)

Breaking Changes

None. The removed enums and unvalidated create_message behavior were introduced in PR #628, which has not been released.

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

[alexhancock]

Nice!