Remove unused mcpserver.exceptions.ValidationError

Never raised in v1 or v2, and it shadows pydantic.ValidationError.

No-Verification-Needed: user opted out of review; dead-code deletion

Author: maxisbey

src/mcp/server/mcpserver/exceptions.py

@@ -5,10 +5,6 @@ class MCPServerError(Exception):
     """Base error for MCPServer."""
 
 
-class ValidationError(MCPServerError):
-    """Error in validating parameters or return values."""
-
-
 class ResourceError(MCPServerError):
     """Error in resource operations."""