fix: return 4xx for non-existent resources in GET requests (#1635)

# What does this PR do?

- Removed Optional return types for GET methods
- Raised ValueError when requested resource is not found
- Ensures proper 4xx response for missing resources
- Updated the API generator to check for wrong signatures

```
$ uv run --with ".[dev]" ./docs/openapi_generator/run_openapi_generator.sh
Validating API method return types...

API Method Return Type Validation Errors:

Method ScoringFunctions.get_scoring_function returns Optional type
```

Closes: https://github.com/meta-llama/llama-stack/issues/1630

## Test Plan

Run the server then:

```
curl http://127.0.0.1:8321/v1/models/foo     
{"detail":"Invalid value: Model 'foo' not found"}%  
```

Server log:

```
INFO:     127.0.0.1:52307 - "GET /v1/models/foo HTTP/1.1" 400 Bad Request
09:51:42.654 [END] /v1/models/foo [StatusCode.OK] (134.65ms)
 09:51:42.651 [ERROR] Error executing endpoint route='/v1/models/{model_id:path}' method='get'
Traceback (most recent call last):
  File "/Users/leseb/Documents/AI/llama-stack/llama_stack/distribution/server/server.py", line 193, in endpoint
    return await maybe_await(value)
  File "/Users/leseb/Documents/AI/llama-stack/llama_stack/distribution/server/server.py", line 156, in maybe_await
    return await value
  File "/Users/leseb/Documents/AI/llama-stack/llama_stack/providers/utils/telemetry/trace_protocol.py", line 102, in async_wrapper
    result = await method(self, *args, **kwargs)
  File "/Users/leseb/Documents/AI/llama-stack/llama_stack/distribution/routers/routing_tables.py", line 217, in get_model
    raise ValueError(f"Model '{model_id}' not found")
ValueError: Model 'foo' not found
```

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/apis/models/models.py

@@ -66,7 +66,7 @@ class Models(Protocol):
     async def get_model(
         self,
         model_id: str,
-    ) -> Optional[Model]: ...
+    ) -> Model: ...
 
     @webmethod(route="/models", method="POST")
     async def register_model(