feat(submit): confirm defaults instead of removing them

Restore defaults for queue/executor_memory/executor_cores/num_executors
in prepare_submit_job, but require the caller to explicitly confirm
them. If any defaulted field is omitted, the route returns HTTP 400
listing the defaults and asks the caller to resubmit with explicit
values.

app_name remains required (no meaningful default). extra_args remains
optional.

Tests cover rejection of unconfirmed defaults and acceptance of
explicitly confirmed defaults.

tests/integration/test_mcp_routes.py

@@ -104,6 +104,54 @@ def test_prepare_submit_job_works_via_body(tmp_path):
     assert body["parameters"]["master"] == "yarn"  # snapshotted from connection
 
 
+def test_prepare_rejects_unconfirmed_defaults(tmp_path):
+    c = TestClient(app)
+    c.post("/save_connection", json={"name": "prod", "master": "yarn"})
+    script = tmp_path / "demo.py"
+    script.write_text("print('hi')\n")
+    r = c.post(
+        "/prepare_submit_job",
+        json={
+            "connection": "prod",
+            "script_path": str(script),
+            "app_name": "test-app",
+        },
+    )
+    assert r.status_code == 400
+    detail = r.json()["detail"]
+    assert "Please confirm default values" in detail
+    assert "queue='default'" in detail
+    assert "executor_memory='4G'" in detail
+    assert "executor_cores=2" in detail
+    assert "num_executors=2" in detail
+
+
+def test_prepare_accepts_confirmed_defaults(tmp_path):
+    c = TestClient(app)
+    c.post("/save_connection", json={"name": "prod", "master": "yarn"})
+    script = tmp_path / "demo.py"
+    script.write_text("print('hi')\n")
+    r = c.post(
+        "/prepare_submit_job",
+        json={
+            "connection": "prod",
+            "script_path": str(script),
+            "queue": "default",
+            "executor_memory": "4G",
+            "executor_cores": 2,
+            "num_executors": 2,
+            "app_name": "test-app",
+        },
+    )
+    assert r.status_code == 200
+    body = r.json()
+    assert body["status"] == "PENDING"
+    assert body["parameters"]["queue"] == "default"
+    assert body["parameters"]["executor_memory"] == "4G"
+    assert body["parameters"]["executor_cores"] == 2
+    assert body["parameters"]["num_executors"] == 2
+
+
 def test_prepare_rejects_nonexistent_script_path_with_400():
     """MCP clients should see a clean 400 with a remediation hint, not a 500."""
     c = TestClient(app)