test: cover YarnError -> 502 across all YARN-touching tools

In 3b698e1 I added the YarnError exception handler and tested it
against get_job_status only. The other four YARN-touching tools
(get_external_job_status, get_external_job_result,
get_external_job_logs, list_applications) were assumed to be
covered by the same handler but never actually exercised against a
YarnError.

Add one integration test per remaining tool. Each one:
  - saves a Connection
  - mocks the underlying yarn_client symbol the tool uses
    (get_application_status / get_application_logs /
    list_applications_yarn) to raise YarnError with a distinctive
    message
  - hits the tool's route via TestClient
  - asserts HTTP 502 + the YarnError message in the response detail

The new tests prove the YarnError -> 502 contract holds uniformly:
  - test_external_job_status_returns_502_on_yarn_error
    (get_application_status raising "not found")
  - test_external_job_result_returns_502_on_yarn_error
    (get_application_status raising "Connection refused")
  - test_external_job_logs_returns_502_on_yarn_error
    (get_application_logs raising "HTTP 500 cluster overloaded")
  - test_list_applications_returns_502_on_yarn_error
    (list_applications_yarn raising "HTTP 503")

Each one uses a different YARN exception message so the test
distinguishes which code path produced the error. Combined with
the existing get_job_status test, the YarnError -> 502 contract
is now verified end-to-end for all five YARN REST-call sites.

Tests: 405 passed (was 401, +4 net).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/integration/test_mcp_routes.py

@@ -579,3 +579,113 @@ def test_yarn_error_returns_502_with_detail(tmp_path, monkeypatch):
     detail = r.json()["detail"]
     assert "YARN connection failed" in detail
     assert "Connection refused" in detail
+
+
+def test_external_job_status_returns_502_on_yarn_error(tmp_path):
+    """get_external_job_status uses get_application_status under the
+    hood — same handler, same 502, same detail."""
+    from unittest.mock import patch
+    from spark_executor.core.yarn_client import YarnError
+
+    connection_store.store.save(
+        Connection(
+            name="prod",
+            master="yarn",
+            yarn_rm_url="http://ccam1:8088",
+        )
+    )
+    c = TestClient(app)
+    with patch(
+        "spark_executor.tools.external_jobs.get_application_status",
+        side_effect=YarnError("YARN application 'application_xxx' not found"),
+    ):
+        r = c.post(
+            "/get_external_job_status",
+            json={"application_id": "application_1740000000001_0001", "connection_name": "prod"},
+        )
+    assert r.status_code == 502
+    assert "not found" in r.json()["detail"]
+
+
+def test_external_job_result_returns_502_on_yarn_error(tmp_path):
+    """get_external_job_result also uses get_application_status (same
+    underlying YARN endpoint), and exercises the same YarnError path."""
+    from unittest.mock import patch
+    from spark_executor.core.yarn_client import YarnError
+
+    connection_store.store.save(
+        Connection(
+            name="prod",
+            master="yarn",
+            yarn_rm_url="http://ccam1:8088",
+        )
+    )
+    c = TestClient(app)
+    with patch(
+        "spark_executor.tools.external_jobs.get_application_status",
+        side_effect=YarnError("YARN connection failed: ConnectError: Connection refused"),
+    ):
+        r = c.post(
+            "/get_external_job_result",
+            json={"application_id": "application_1740000000001_0001", "connection_name": "prod"},
+        )
+    assert r.status_code == 502
+    assert "YARN connection failed" in r.json()["detail"]
+
+
+def test_external_job_logs_returns_502_on_yarn_error(tmp_path):
+    """get_external_job_logs uses get_application_logs — different
+    function, but raises YarnError the same way. Handler must catch
+    it the same way."""
+    from unittest.mock import patch
+    from spark_executor.core.yarn_client import YarnError
+
+    connection_store.store.save(
+        Connection(
+            name="prod",
+            master="yarn",
+            yarn_rm_url="http://ccam1:8088",
+        )
+    )
+    c = TestClient(app)
+    with patch(
+        "spark_executor.tools.external_jobs.get_application_logs",
+        side_effect=YarnError("YARN GET logs returned HTTP 500: cluster overloaded"),
+    ):
+        r = c.post(
+            "/get_external_job_logs",
+            json={
+                "application_id": "application_1740000000001_0001",
+                "connection_name": "prod",
+                "tail_chars": 5000,
+            },
+        )
+    assert r.status_code == 502
+    assert "YARN GET logs returned HTTP 500" in r.json()["detail"]
+    assert "cluster overloaded" in r.json()["detail"]
+
+
+def test_list_applications_returns_502_on_yarn_error(tmp_path):
+    """list_applications uses list_applications_yarn — yet another
+    YARN-touching tool. Same YarnError -> 502 contract."""
+    from unittest.mock import patch
+    from spark_executor.core.yarn_client import YarnError
+
+    connection_store.store.save(
+        Connection(
+            name="prod",
+            master="yarn",
+            yarn_rm_url="http://ccam1:8088",
+        )
+    )
+    c = TestClient(app)
+    with patch(
+        "spark_executor.tools.external_jobs.list_applications_yarn",
+        side_effect=YarnError("YARN list applications failed: 503 Service Unavailable"),
+    ):
+        r = c.post(
+            "/list_applications",
+            json={"connection_name": "prod"},
+        )
+    assert r.status_code == 502
+    assert "YARN list applications failed" in r.json()["detail"]