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/core/routers/safety.py

@@ -10,6 +10,7 @@ from llama_stack.apis.inference import Message
 from llama_stack.apis.safety import RunShieldResponse, Safety
 from llama_stack.apis.safety.safety import ModerationObject
 from llama_stack.apis.shields import Shield
+from llama_stack.core.datatypes import SafetyConfig
 from llama_stack.log import get_logger
 from llama_stack.providers.datatypes import RoutingTable
 
@@ -20,9 +21,11 @@ class SafetyRouter(Safety):
     def __init__(
         self,
         routing_table: RoutingTable,
+        safety_config: SafetyConfig | None = None,
     ) -> None:
         logger.debug("Initializing SafetyRouter")
         self.routing_table = routing_table
+        self.safety_config = safety_config
 
     async def initialize(self) -> None:
         logger.debug("SafetyRouter.initialize")
@@ -60,30 +63,47 @@ class SafetyRouter(Safety):
             params=params,
         )
 
-    async def run_moderation(self, input: str | list[str], model: str) -> ModerationObject:
-        async def get_shield_id(self, model: str) -> str:
-            """Get Shield id from model (provider_resource_id) of shield."""
-            list_shields_response = await self.routing_table.list_shields()
+    async def run_moderation(self, input: str | list[str], model: str | None = None) -> ModerationObject:
+        list_shields_response = await self.routing_table.list_shields()
+        shields = list_shields_response.data
 
-            matches: list[str] = [s.identifier for s in list_shields_response.data if model == s.provider_resource_id]
+        selected_shield: Shield | None = None
+        provider_model: str | None = model
 
+        if model:
+            matches: list[Shield] = [s for s in shields if model == s.provider_resource_id]
             if not matches:
                 raise ValueError(
-                    f"No shield associated with provider_resource id {model}: choose from {[s.provider_resource_id for s in list_shields_response.data]}"
+                    f"No shield associated with provider_resource id {model}: choose from {[s.provider_resource_id for s in shields]}"
                 )
             if len(matches) > 1:
                 raise ValueError(
-                    f"Multiple shields associated with provider_resource id {model}: matched shields {matches}"
+                    f"Multiple shields associated with provider_resource id {model}: matched shields {[s.identifier for s in matches]}"
+                )
+            selected_shield = matches[0]
+        else:
+            default_shield_id = self.safety_config.default_shield_id if self.safety_config else None
+            if not default_shield_id:
+                raise ValueError(
+                    "No moderation model specified and no default_shield_id configured in safety config: select model "
+                    f"from {[s.provider_resource_id or s.identifier for s in shields]}"
                 )
-            return matches[0]
 
-        shield_id = await get_shield_id(self, model)
+            selected_shield = next((s for s in shields if s.identifier == default_shield_id), None)
+            if selected_shield is None:
+                raise ValueError(
+                    f"Default moderation model not found. Choose from {[s.provider_resource_id or s.identifier for s in shields]}."
+                )
+
+            provider_model = selected_shield.provider_resource_id
+
+        shield_id = selected_shield.identifier
         logger.debug(f"SafetyRouter.run_moderation: {shield_id}")
         provider = await self.routing_table.get_provider_impl(shield_id)
 
         response = await provider.run_moderation(
             input=input,
-            model=model,
+            model=provider_model,
         )
 
         return response