Cleanup: ReplaceOldSchemaMockMcp with MockMcp in tests

[kpavlov]

Replace OldSchemaMockMcp with MockMcp in tests and bump mokksy dependency to 0.6.2.

Motivation and Context

Test cleanup

How Has This Been Tested?

Regression tests

Breaking Changes

No

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
• Test 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