chore: support default model in moderations API

# What does this PR do?


## Test Plan

llama_stack/distributions/ci-tests/run.yaml

@@ -274,3 +274,5 @@ vector_stores:
   default_embedding_model:
     provider_id: sentence-transformers
     model_id: nomic-ai/nomic-embed-text-v1.5
+safety:
+  default_shield_id: llama-guard

llama_stack/distributions/starter-gpu/run.yaml

@@ -277,3 +277,5 @@ vector_stores:
   default_embedding_model:
     provider_id: sentence-transformers
     model_id: nomic-ai/nomic-embed-text-v1.5
+safety:
+  default_shield_id: llama-guard

llama_stack/distributions/starter/run.yaml

@@ -274,3 +274,5 @@ vector_stores:
   default_embedding_model:
     provider_id: sentence-transformers
     model_id: nomic-ai/nomic-embed-text-v1.5
+safety:
+  default_shield_id: llama-guard

llama_stack/distributions/starter/starter.py

@@ -12,6 +12,7 @@ from llama_stack.core.datatypes import (
     Provider,
     ProviderSpec,
     QualifiedModel,
+    SafetyConfig,
     ShieldInput,
     ToolGroupInput,
     VectorStoresConfig,
@@ -256,6 +257,9 @@ def get_distribution_template(name: str = "starter") -> DistributionTemplate:
                         model_id="nomic-ai/nomic-embed-text-v1.5",
                     ),
                 ),
+                safety_config=SafetyConfig(
+                    default_shield_id="llama-guard",
+                ),
             ),
         },
         run_config_env_vars={

llama_stack/distributions/template.py

@@ -24,6 +24,7 @@ from llama_stack.core.datatypes import (
     DistributionSpec,
     ModelInput,
     Provider,
+    SafetyConfig,
     ShieldInput,
     TelemetryConfig,
     ToolGroupInput,
@@ -188,6 +189,7 @@ class RunConfigSettings(BaseModel):
     default_datasets: list[DatasetInput] | None = None
     default_benchmarks: list[BenchmarkInput] | None = None
     vector_stores_config: VectorStoresConfig | None = None
+    safety_config: SafetyConfig | None = None
     telemetry: TelemetryConfig = Field(default_factory=lambda: TelemetryConfig(enabled=True))
     storage_backends: dict[str, Any] | None = None
     storage_stores: dict[str, Any] | None = None
@@ -290,6 +292,9 @@ class RunConfigSettings(BaseModel):
         if self.vector_stores_config:
             config["vector_stores"] = self.vector_stores_config.model_dump(exclude_none=True)
 
+        if self.safety_config:
+            config["safety"] = self.safety_config.model_dump(exclude_none=True)
+
         return config