fix: typo MCP_SESSION_ID, keep consistent style with other identifiers

[JunJieLiu51520]

Motivation and Context

How Has This Been Tested?

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

[JunJieLiu51520]

@tzolov PTAL

[tzolov]

Thanks @JunJieLiu51520
FYI, HTTP headers are case-insensitive. Therefore this is just a stylish not functional issue.