Error handling added to MVP according to error_handling.md guideline

tests/services/test_transcription.py

@@ -60,9 +60,12 @@ class TestPromptLoading:
         prompt_dir.mkdir()
 
         settings = Settings(openrouter_api_key="test-key", prompt_dir=prompt_dir)
-        with pytest.raises(PromptLoadError):
+        with pytest.raises(PromptLoadError) as exc_info:
             load_prompt_text(settings=settings)
 
+        assert exc_info.value.category.value == "infrastructure_persistent_error"
+        assert "verify prompt_dir" in exc_info.value.suggestion.lower()
+
 
 @pytest.mark.unit
 class TestImageLoading:
@@ -83,9 +86,12 @@ class TestImageLoading:
         """Image loader raises TranscriptionError when image file does not exist."""
         missing = tmp_path / "missing.png"
 
-        with pytest.raises(TranscriptionError):
+        with pytest.raises(TranscriptionError) as exc_info:
             load_image_payload(missing)
 
+        assert exc_info.value.category.value == "not_found_error"
+        assert "verify" in exc_info.value.suggestion.lower()
+
 
 @pytest.mark.unit
 class TestTranscriptionService:
@@ -123,5 +129,9 @@ class TestTranscriptionService:
         settings = Settings(openrouter_api_key="test-key", prompt_dir=prompt_dir)
         provider = _FakeProvider(error=ProviderError("upstream failure"))
 
-        with pytest.raises(TranscriptionError):
+        with pytest.raises(TranscriptionError) as exc_info:
             transcribe_document_image(image_path, settings=settings, provider=provider)
+
+        assert exc_info.value.category.value == "external_provider_error"
+        assert exc_info.value.retriable is True
+        assert "retry" in exc_info.value.suggestion.lower()