chore: Improve error message for missing provider dependencies (#3315)

Generated with CC:

Replace cryptic KeyError with clear, actionable error message that
shows:
- Which API the failing provider belongs to
- The provider ID and type that's failing
- Which dependency is missing
- Clear instructions on how to fix the issue


## Test plan
Use a run config with Agents API and no safety provider

Before: KeyError: <Api.safety: 'safety'>
After: Failed to resolve 'agents' provider 'meta-reference' of type
'inline::meta-reference': required dependency 'safety' is not available.
Please add a 'safety' provider to your configuration or check if the
provider is properly configured.

llama_stack/core/resolver.py

@@ -284,7 +284,15 @@ async def instantiate_providers(
         if provider.provider_id is None:
             continue
 
+        try:
             deps = {a: impls[a] for a in provider.spec.api_dependencies}
+        except KeyError as e:
+            missing_api = e.args[0]
+            raise RuntimeError(
+                f"Failed to resolve '{provider.spec.api.value}' provider '{provider.provider_id}' of type '{provider.spec.provider_type}': "
+                f"required dependency '{missing_api.value}' is not available. "
+                f"Please add a '{missing_api.value}' provider to your configuration or check if the provider is properly configured."
+            ) from e
         for a in provider.spec.optional_api_dependencies:
             if a in impls:
                 deps[a] = impls[a]