fix: allow use of models registered at runtime (#1980)

# What does this PR do?

fix a bug where models registered at runtime could not be used.

```
$ llama-stack-client models register test-model --provider-id nvidia --provider-model-id meta/llama-3.1-70b-instruct

$ curl http://localhost:8321/v1/openai/v1/chat/completions \                                                        
-H "Content-Type: application/json" \
-d '{
  "model": "test-model",
  "messages": [{"role": "user", "content": "What is the weather like in Boston today?"}]
}'

=(client)=> {"detail":"Internal server error: An unexpected error occurred."}
=(server)=> TypeError: Missing required arguments; Expected either ('messages' and 'model') or ('messages', 'model' and 'stream') arguments to be given
```

*root cause:* test-model is not added to ModelRegistryHelper's
alias_to_provider_id_map.

as part of the fix, this adds tests for ModelRegistryHelper and defines
its expected behavior.

user visible behavior changes -

| action | existing behavior | new behavior |
| -- | -- | -- |
| double register | success (but no change) | error |
| register unknown | success (fail when used) | error |

existing behavior for register unknown model and double register -
```
$ llama-stack-client models register test-model --provider-id nvidia --provider-model-id meta/llama-3.1-70b-instruct-unknown
Successfully registered model test-model

$ llama-stack-client models list | grep test-model
│ llm │ test-model                               │ meta/llama-3.1-70b-instruct-unknown │     │ nv… │

$ llama-stack-client models register test-model --provider-id nvidia --provider-model-id meta/llama-3.1-70b-instruct       
Successfully registered model test-model

$ llama-stack-client models list | grep test-model
│ llm │ test-model                               │ meta/llama-3.1-70b-instruct-unknown │     │ nv… │
```

new behavior for register unknown -
```
$ llama-stack-client models register test-model --provider-id nvidia --provider-model-id meta/llama-3.1-70b-instruct-unknown
╭──────────────────────────────────────────────────────────────────────────────────────────────────╮
│ Failed to register model                                                                         │
│                                                                                                  │
│ Error Type: BadRequestError                                                                      │
│ Details: Error code: 400 - {'detail': "Invalid value: Model id                                   │
│ 'meta/llama-3.1-70b-instruct-unknown' is not supported. Supported ids are:                       │
│ meta/llama-3.1-70b-instruct, snowflake/arctic-embed-l, meta/llama-3.2-1b-instruct,               │
│ nvidia/nv-embedqa-mistral-7b-v2, meta/llama-3.2-90b-vision-instruct, meta/llama-3.2-3b-instruct, │
│ meta/llama-3.2-11b-vision-instruct, meta/llama-3.1-405b-instruct, meta/llama3-8b-instruct,       │
│ meta/llama3-70b-instruct, nvidia/llama-3.2-nv-embedqa-1b-v2, meta/llama-3.1-8b-instruct,         │
│ nvidia/nv-embedqa-e5-v5"}                                                                        │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
```

new behavior for double register -
```
$ llama-stack-client models register test-model --provider-id nvidia --provider-model-id meta/llama-3.1-70b-instruct
Successfully registered model test-model

$ llama-stack-client models register test-model --provider-id nvidia --provider-model-id meta/llama-3.2-1b-instruct 
╭──────────────────────────────────────────────────────────────────────────────────────────────────╮
│ Failed to register model                                                                         │
│                                                                                                  │
│ Error Type: BadRequestError                                                                      │
│ Details: Error code: 400 - {'detail': "Invalid value: Model id 'test-model' is already           │
│ registered. Please use a different id or unregister it first."}                                  │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
```


## Test Plan

```
uv run pytest -v tests/unit/providers/utils/test_model_registry.py
```

llama_stack/providers/remote/inference/vllm/vllm.py

@@ -372,7 +372,10 @@ class VLLMInferenceAdapter(Inference, ModelsProtocolPrivate):
         # self.client should only be created after the initialization is complete to avoid asyncio cross-context errors.
         # Changing this may lead to unpredictable behavior.
         client = self._create_client() if self.client is None else self.client
-        model = await self.register_helper.register_model(model)
+        try:
+            model = await self.register_helper.register_model(model)
+        except ValueError:
+            pass  # Ignore statically unknown model, will check live listing
         res = await client.models.list()
         available_models = [m.id async for m in res]
         if model.provider_resource_id not in available_models: