fix: Fixing prompts import warning (#3455)

# What does this PR do?
Fixes this warning in llama stack build:

```bash
WARNING  2025-09-15 15:29:02,197 llama_stack.core.distribution:149 core: Failed to import module prompts: No module named
         'llama_stack.providers.registry.prompts'"
```

## Test Plan
Test added

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>

llama_stack/core/distribution.py

@@ -26,6 +26,9 @@ from llama_stack.providers.datatypes import (
 logger = get_logger(name=__name__, category="core")
 
 
+INTERNAL_APIS = {Api.inspect, Api.providers, Api.prompts}
+
+
 def stack_apis() -> list[Api]:
     return list(Api)
 
@@ -70,7 +73,7 @@ def builtin_automatically_routed_apis() -> list[AutoRoutedApiInfo]:
 
 def providable_apis() -> list[Api]:
     routing_table_apis = {x.routing_table_api for x in builtin_automatically_routed_apis()}
-    return [api for api in Api if api not in routing_table_apis and api != Api.inspect and api != Api.providers]
+    return [api for api in Api if api not in routing_table_apis and api not in INTERNAL_APIS]
 
 
 def _load_remote_provider_spec(spec_data: dict[str, Any], api: Api) -> ProviderSpec:

tests/unit/distribution/test_distribution.py

@@ -12,7 +12,7 @@ import yaml
 from pydantic import BaseModel, Field, ValidationError
 
 from llama_stack.core.datatypes import Api, Provider, StackRunConfig
-from llama_stack.core.distribution import get_provider_registry
+from llama_stack.core.distribution import INTERNAL_APIS, get_provider_registry, providable_apis
 from llama_stack.providers.datatypes import ProviderSpec
 
 
@@ -152,6 +152,24 @@ class TestProviderRegistry:
         assert registry[Api.inference]["test_provider"].provider_type == "test_provider"
         assert registry[Api.inference]["test_provider"].api == Api.inference
 
+    def test_internal_apis_excluded(self):
+        """Test that internal APIs are excluded and APIs without provider registries are marked as internal."""
+        import importlib
+
+        apis = providable_apis()
+
+        for internal_api in INTERNAL_APIS:
+            assert internal_api not in apis, f"Internal API {internal_api} should not be in providable_apis"
+
+        for api in apis:
+            module_name = f"llama_stack.providers.registry.{api.name.lower()}"
+            try:
+                importlib.import_module(module_name)
+            except ImportError as err:
+                raise AssertionError(
+                    f"API {api} is in providable_apis but has no provider registry module ({module_name})"
+                ) from err
+
     def test_external_remote_providers(self, api_directories, mock_providers, base_config, provider_spec_yaml):
         """Test loading external remote providers from YAML files."""
         remote_dir, _ = api_directories