fix: map KeyError->404 and ValueError->400 for MCP clients

Tool functions raise KeyError for unknown ids and ValueError for invalid
state transitions. Without handlers, FastAPI returned a bare 500
"Internal Server Error" to MCP clients — useless for an agent that needs
to know whether the id was wrong vs the cluster was down.

KeyError -> 404 with the message ("Unknown job_id: foo").
ValueError -> 400 with the message (e.g. "pending_id p_xyz is in status
'CANCELLED', not PENDING").

spark_executor/server.py

@@ -3,7 +3,8 @@
 @Time   :2026/6/24
 @Author :tao.chen
 """
-from fastapi import FastAPI
+from fastapi import FastAPI, Request
+from fastapi.responses import JSONResponse
 
 from spark_executor.tools.connections import (
     delete_connection,
@@ -34,6 +35,23 @@ from spark_executor.tools.submit import (
 app = FastAPI(title="Spark Executor", version="0.0.1", description="Spark Executor MCP Server")
 
 
+# --- Exception handlers: translate tool-layer errors into proper HTTP statuses ---
+#
+# Tool functions raise KeyError for "unknown id" (job_id, pending_id, connection
+# name) and ValueError for invalid state transitions (e.g. confirming a
+# CANCELLED pending). Without these handlers FastAPI would map them to a bare
+# 500 "Internal Server Error" which is useless to MCP clients.
+
+@app.exception_handler(KeyError)
+async def _keyerror_handler(_request: Request, exc: KeyError) -> JSONResponse:
+    return JSONResponse(status_code=404, content={"detail": str(exc)})
+
+
+@app.exception_handler(ValueError)
+async def _valueerror_handler(_request: Request, exc: ValueError) -> JSONResponse:
+    return JSONResponse(status_code=400, content={"detail": str(exc)})
+
+
 @app.get("/health")
 def health_check():
     return {"status": "ok"}