chore: support default model in moderations API (#3890)

# What does this PR do?
https://platform.openai.com/docs/api-reference/moderations supports
optional model parameter.

This PR adds support for using moderations API with model=None if a
default shield id is provided via safety config.

## Test Plan
added tests

manual test:
```
> SAFETY_MODEL='together/meta-llama/Llama-Guard-4-12B'   uv run llama stack run starter
> curl http://localhost:8321/v1/moderations \
  -H "Content-Type: application/json" \
  -d '{
    "input": [
        "hello"
    ]
  }'
```

docs/static/deprecated-llama-stack-spec.html

@@ -8185,13 +8185,12 @@
                     },
                     "model": {
                         "type": "string",
-                        "description": "The content moderation model you would like to use."
+                        "description": "(Optional) The content moderation model you would like to use."
                     }
                 },
                 "additionalProperties": false,
                 "required": [
-                    "input",
-                    "model"
+                    "input"
                 ],
                 "title": "RunModerationRequest"
             },

docs/static/deprecated-llama-stack-spec.yaml

@@ -6104,11 +6104,10 @@ components:
         model:
           type: string
           description: >-
-            The content moderation model you would like to use.
+            (Optional) The content moderation model you would like to use.
       additionalProperties: false
       required:
         - input
-        - model
       title: RunModerationRequest
     ModerationObject:
       type: object

docs/static/llama-stack-spec.html

@@ -6919,13 +6919,12 @@
                     },
                     "model": {
                         "type": "string",
-                        "description": "The content moderation model you would like to use."
+                        "description": "(Optional) The content moderation model you would like to use."
                     }
                 },
                 "additionalProperties": false,
                 "required": [
-                    "input",
-                    "model"
+                    "input"
                 ],
                 "title": "RunModerationRequest"
             },

docs/static/llama-stack-spec.yaml

@@ -5230,11 +5230,10 @@ components:
         model:
           type: string
           description: >-
-            The content moderation model you would like to use.
+            (Optional) The content moderation model you would like to use.
       additionalProperties: false
       required:
         - input
-        - model
       title: RunModerationRequest
     ModerationObject:
       type: object

docs/static/stainless-llama-stack-spec.html

@@ -8591,13 +8591,12 @@
                     },
                     "model": {
                         "type": "string",
-                        "description": "The content moderation model you would like to use."
+                        "description": "(Optional) The content moderation model you would like to use."
                     }
                 },
                 "additionalProperties": false,
                 "required": [
-                    "input",
-                    "model"
+                    "input"
                 ],
                 "title": "RunModerationRequest"
             },

docs/static/stainless-llama-stack-spec.yaml

@@ -6443,11 +6443,10 @@ components:
         model:
           type: string
           description: >-
-            The content moderation model you would like to use.
+            (Optional) The content moderation model you would like to use.
       additionalProperties: false
       required:
         - input
-        - model
       title: RunModerationRequest
     ModerationObject:
       type: object