fix(rmcp): reject duplicate SEP-2243 headers

[DaleSeo]

Fixes #1272.

Motivation and Context

For SEP-2243, header validation used HeaderMap::get to read each header. That method returns only the first of several field lines with the same name. As a result, a request with two conflicting Mcp-Method values was validated against the first value, while the second was silently ignored. header_str now returns Result<Option<&str>, String> and rejects duplicate field lines. This handles Mcp-Method, Mcp-Name, and Mcp-Param-* in one place.

How Has This Been Tested?

Added tests

Breaking Changes

None

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