SIENTIAPDE-2226: Add plugin store e2e docs and cover repo edge cases

- Document PluginStore wiring e2e scenarios (4.1.1-4.1.5) covering env
  config, explicit branch, missing model, bad repo and bad credentials.
- Add unit tests for DataManagerRepository date parsing, timestamp
  dropping, numeric coercion and timezone handling to keep coverage at
  100%.
- Mark the cleanup guard in TrainModel as `no branch`, since the false
  branch always exits through the re-raise.

model_manager/workflows/train_model.py

@@ -131,7 +131,9 @@ class TrainModel:
             training_succeeded = True
         finally:
             try:
-                if train_result is not None:
+                # train_result is only None when training raised, so the false branch
+                # always leaves through the re-raise and never reaches the return below.
+                if train_result is not None:  # pragma: no branch
                     await self._cleanup_resources(
                         run_dir=train_result.get('run_dir'),
                         metadata=metadata,