feat: raise ValueError when no stdout source available in from_completed_process

- Replace empty EntrypointOutput creation with ValueError
- Add clear error message when both stdout_file and completed_process.stdout are None
- Improves error handling and prevents silent failures
- Addresses GitHub PR feedback from @aaronsteers

Co-Authored-By: AJ Steers <[email protected]>

Author: devin-ai-integration[bot]

airbyte_cdk/test/entrypoint_wrapper.py

@@ -306,7 +306,9 @@ def from_completed_process(
             messages = completed_process.stdout.splitlines()
             instance = cls(messages=messages)
         else:
-            instance = cls()
+            raise ValueError(
+                "Either stdout_file should be provided or completed_process.stdout should not be None"
+            )
 
         return instance