test(ocr): narrow training error test to subprocess.run seam

The asyncio.to_thread patch stubbed out the entire _run_training call,
hiding the real error path. Replacing it with a failing CompletedProcess
from subprocess.run exercises the actual ketos-failed branch and keeps
the test's intent — error counter bumps, 500 surfaces — intact.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ocr-service/test_metrics.py

@@ -347,13 +347,21 @@ async def test_ocr_training_runs_total_incremented_with_recognition_success_labe
 
 @pytest.mark.asyncio
 async def test_ocr_training_runs_total_incremented_with_recognition_error_label(fresh_metrics):
-    """When /train's inner runner raises, the error counter bumps and the exception propagates."""
-    async def fake_to_thread(func, *args, **kwargs):
-        raise RuntimeError("ketos train failed (exit 1): synthetic")
+    """When ketos exits non-zero, the error counter bumps and the exception propagates.
+
+    Uses the narrowest available seam — `subprocess.run` returning a failing
+    CompletedProcess — instead of stubbing the asyncio.to_thread boundary,
+    so the test exercises the real _run_training error path.
+    """
+    from subprocess import CompletedProcess
+
+    failing_proc = CompletedProcess(
+        args=["ketos"], returncode=1, stdout="", stderr="synthetic ketos failure"
+    )
 
     with patch("main.TRAINING_TOKEN", "secret-token"), \
          patch("main._models_ready", True), \
-         patch("main.asyncio.to_thread", side_effect=fake_to_thread):
+         patch("main.subprocess.run", return_value=failing_proc):
         transport = ASGITransport(app=app, raise_app_exceptions=False)
         async with AsyncClient(transport=transport, base_url="http://test") as client:
             response = await client.post(