Use TypedDict for OAuth error responses

Replace Dict[str, str] with a more precise TypedDict that captures the exact
structure of OAuth error responses. Also updates imports to use
Python 3.10 style type annotations.

Author: jerome3o-anthropic

src/mcp/server/auth/errors.py

@@ -4,7 +4,13 @@
 Corresponds to TypeScript file: src/server/auth/errors.ts
 """
 
-from typing import Dict, Optional, Any
+from typing import Any, TypedDict
+
+
+class OAuthErrorResponse(TypedDict):
+    """OAuth error response format."""
+    error: str
+    error_description: str
 
 
 class OAuthError(Exception):
@@ -19,7 +25,7 @@ def __init__(self, message: str):
         super().__init__(message)
         self.message = message
 
-    def to_response_object(self) -> Dict[str, str]:
+    def to_response_object(self) -> OAuthErrorResponse:
         """Convert error to JSON response object."""
         return {
             "error": self.error_code,