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"
    ]
  }'
```

llama_stack/apis/safety/safety.py

@@ -123,13 +123,13 @@ class Safety(Protocol):
 
     @webmethod(route="/openai/v1/moderations", method="POST", level=LLAMA_STACK_API_V1, deprecated=True)
     @webmethod(route="/moderations", method="POST", level=LLAMA_STACK_API_V1)
-    async def run_moderation(self, input: str | list[str], model: str) -> ModerationObject:
+    async def run_moderation(self, input: str | list[str], model: str | None = None) -> ModerationObject:
         """Create moderation.
 
         Classifies if text and/or image inputs are potentially harmful.
         :param input: Input (or inputs) to classify.
         Can be a single string, an array of strings, or an array of multi-modal input objects similar to other models.
-        :param model: The content moderation model you would like to use.
+        :param model: (Optional) The content moderation model you would like to use.
         :returns: A moderation object.
         """
         ...