fix(ocr): narrow exception handling and add unit tests for ensure_blla_model

- _model_is_loadable: narrow bare except to (RuntimeError, OSError, ValueError)
  with DEBUG-level fallback for unexpected exceptions — prevents silent masking
  of missing kraken install or AttributeError on vgsl
- _run_segtrain: replace bare except:pass with log.warning so height-check
  fallback is visible in container logs
- New test_ensure_blla_model.py: covers model-OK early return, incompatible
  model rename+replace, and missing model download paths

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

ocr-service/test_ensure_blla_model.py

@@ -0,0 +1,69 @@
+"""Unit tests for ensure_blla_model.main()."""
+
+from unittest.mock import MagicMock, call, patch
+
+import ensure_blla_model
+
+
+# ─── Model already loadable ───────────────────────────────────────────────────
+
+
+def test_main_returns_early_when_model_is_loadable():
+    """When the model exists and loads cleanly, no download or rename occurs."""
+    with (
+        patch("os.path.exists", return_value=True),
+        patch.object(ensure_blla_model, "_model_is_loadable", return_value=True),
+        patch.object(ensure_blla_model, "_download_blla") as mock_download,
+        patch("os.rename") as mock_rename,
+    ):
+        ensure_blla_model.main()
+
+    mock_download.assert_not_called()
+    mock_rename.assert_not_called()
+
+
+# ─── Model exists but is incompatible ─────────────────────────────────────────
+
+
+def test_main_replaces_incompatible_model():
+    """An incompatible model is renamed and replaced with a fresh download."""
+    fake_path = "/app/models/blla.mlmodel"
+    downloaded_path = "/tmp/downloaded.mlmodel"
+
+    with (
+        patch.object(ensure_blla_model, "BLLA_MODEL_PATH", fake_path),
+        patch("os.path.exists", return_value=True),
+        patch.object(ensure_blla_model, "_model_is_loadable", return_value=False),
+        patch.object(ensure_blla_model, "_download_blla", return_value=downloaded_path),
+        patch("os.rename") as mock_rename,
+        patch("shutil.copy2") as mock_copy,
+        patch("os.makedirs"),
+    ):
+        ensure_blla_model.main()
+
+    mock_rename.assert_called_once_with(fake_path, fake_path + ".incompatible")
+    mock_copy.assert_called_once_with(downloaded_path, fake_path)
+
+
+# ─── Model missing ────────────────────────────────────────────────────────────
+
+
+def test_main_downloads_when_model_missing():
+    """When the model file doesn't exist at all, it is downloaded without rename."""
+    fake_path = "/app/models/blla.mlmodel"
+    downloaded_path = "/tmp/downloaded.mlmodel"
+
+    with (
+        patch.object(ensure_blla_model, "BLLA_MODEL_PATH", fake_path),
+        patch("os.path.exists", return_value=False),
+        patch.object(ensure_blla_model, "_model_is_loadable") as mock_loadable,
+        patch.object(ensure_blla_model, "_download_blla", return_value=downloaded_path),
+        patch("os.rename") as mock_rename,
+        patch("shutil.copy2") as mock_copy,
+        patch("os.makedirs"),
+    ):
+        ensure_blla_model.main()
+
+    mock_loadable.assert_not_called()
+    mock_rename.assert_not_called()
+    mock_copy.assert_called_once_with(downloaded_path, fake_path)