Log prompt validation errors without unnecessary traceback

When a prompt is called without a required argument, the ValueError
is an expected validation failure. Previously it was logged with
logger.exception() which includes a full traceback, cluttering
server logs. Now ValueError is caught separately and logged with
logger.warning() (message only), while truly unexpected exceptions
still get the full traceback via logger.exception().

Fixes #3342

Author: Sachin

src/mcp/server/mcpserver/server.py

@@ -1298,6 +1298,12 @@ async def get_prompt(
             )
         except MCPError:
             raise
+        except ValueError as e:
+            # Expected validation errors (e.g. missing required arguments)
+            # don't need a full traceback — a warning with the message is
+            # enough for server operators to diagnose the problem.
+            logger.warning(f"Error getting prompt {name}: {e}")
+            raise
         except Exception as e:
             logger.exception(f"Error getting prompt {name}")
             raise ValueError(str(e)) from e