feat: reject auth servers lacking S256 PKCE support

[DaleSeo]

Motivation and Context

The client always sends an S256 PKCE challenge, but when an authorization server's metadata advertised code_challenge_methods_supported without S256, validate_server_metadata only warned and continued. So rmcp would start a flow the authorization server can't complete and fail confusingly mid-exchange. It now returns AuthError::PkceUnsupported up front for that case, matching TypeScript SDK. An authorization server that omits the field entirely is still allowed (it usually means the server didn't advertise PKCE, not that it lacks S256).

How Has This Been Tested?

New unit tests cover the matrix.

Breaking Changes

None. Only behavioral change.

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