chore(api): add mypy coverage to apis (#2648)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
This PR adds static type coverage to `llama-stack/apis`

Part of https://github.com/meta-llama/llama-stack/issues/2647

<!-- 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.* -->

Signed-off-by: Mustafa Elbehery <melbeher@redhat.com>

docs/_static/llama-stack-spec.yaml

@@ -7838,8 +7838,30 @@ components:
         - start_time
       title: Trace
     Checkpoint:
-      description: Checkpoint created during training runs
+      type: object
+      properties:
+        identifier:
+          type: string
+        created_at:
+          type: string
+          format: date-time
+        epoch:
+          type: integer
+        post_training_job_id:
+          type: string
+        path:
+          type: string
+        training_metrics:
+          $ref: '#/components/schemas/PostTrainingMetric'
+      additionalProperties: false
+      required:
+        - identifier
+        - created_at
+        - epoch
+        - post_training_job_id
+        - path
       title: Checkpoint
+      description: Checkpoint created during training runs
     PostTrainingJobArtifactsResponse:
       type: object
       properties:
@@ -7855,6 +7877,24 @@ components:
         - checkpoints
       title: PostTrainingJobArtifactsResponse
       description: Artifacts of a finetuning job.
+    PostTrainingMetric:
+      type: object
+      properties:
+        epoch:
+          type: integer
+        train_loss:
+          type: number
+        validation_loss:
+          type: number
+        perplexity:
+          type: number
+      additionalProperties: false
+      required:
+        - epoch
+        - train_loss
+        - validation_loss
+        - perplexity
+      title: PostTrainingMetric
     PostTrainingJobStatusResponse:
       type: object
       properties: