fix: Handle case when Customizer Job status is unknown (#1965)

# What does this PR do?
This PR handles the case where a Customization Job's status is
`unknown`. Since we don't map `unknown` to a valid `JobStatus`, the
PostTraining provider throws an exception when fetching/listing a job.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]
`./scripts/unit-tests.sh
tests/unit/providers/nvidia/test_supervised_fine_tuning.py` succeeds

[//]: # (## Documentation)

Co-authored-by: Jash Gulabrai <jgulabrai@nvidia.com>

llama_stack/providers/remote/post_training/nvidia/post_training.py

@@ -27,11 +27,12 @@ from .models import _MODEL_ENTRIES
 
 # Map API status to JobStatus enum
 STATUS_MAPPING = {
-    "running": "in_progress",
-    "completed": "completed",
-    "failed": "failed",
-    "cancelled": "cancelled",
-    "pending": "scheduled",
+    "running": JobStatus.in_progress.value,
+    "completed": JobStatus.completed.value,
+    "failed": JobStatus.failed.value,
+    "cancelled": JobStatus.cancelled.value,
+    "pending": JobStatus.scheduled.value,
+    "unknown": JobStatus.scheduled.value,
 }