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/benchmarks/benchmarks.py

@@ -52,7 +52,7 @@ class Benchmarks(Protocol):
     async def get_benchmark(
         self,
         benchmark_id: str,
-    ) -> Optional[Benchmark]: ...
+    ) -> Benchmark: ...
 
     @webmethod(route="/eval/benchmarks", method="POST")
     async def register_benchmark(

llama_stack/apis/datasets/datasets.py

@@ -201,7 +201,7 @@ class Datasets(Protocol):
     async def get_dataset(
         self,
         dataset_id: str,
-    ) -> Optional[Dataset]: ...
+    ) -> Dataset: ...
 
     @webmethod(route="/datasets", method="GET")
     async def list_datasets(self) -> ListDatasetsResponse: ...

llama_stack/apis/eval/eval.py

@@ -117,7 +117,7 @@ class Eval(Protocol):
         """
 
     @webmethod(route="/eval/benchmarks/{benchmark_id}/jobs/{job_id}", method="GET")
-    async def job_status(self, benchmark_id: str, job_id: str) -> Optional[JobStatus]:
+    async def job_status(self, benchmark_id: str, job_id: str) -> JobStatus:
         """Get the status of a job.
 
         :param benchmark_id: The ID of the benchmark to run the evaluation on.

llama_stack/apis/files/files.py

@@ -115,7 +115,7 @@ class Files(Protocol):
     async def get_upload_session_info(
         self,
         upload_id: str,
-    ) -> Optional[FileUploadResponse]:
+    ) -> FileUploadResponse:
         """
         Returns information about an existsing upload session
 

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(

llama_stack/apis/post_training/post_training.py

@@ -202,10 +202,10 @@ class PostTraining(Protocol):
     async def get_training_jobs(self) -> ListPostTrainingJobsResponse: ...
 
     @webmethod(route="/post-training/job/status", method="GET")
-    async def get_training_job_status(self, job_uuid: str) -> Optional[PostTrainingJobStatusResponse]: ...
+    async def get_training_job_status(self, job_uuid: str) -> PostTrainingJobStatusResponse: ...
 
     @webmethod(route="/post-training/job/cancel", method="POST")
     async def cancel_training_job(self, job_uuid: str) -> None: ...
 
     @webmethod(route="/post-training/job/artifacts", method="GET")
-    async def get_training_job_artifacts(self, job_uuid: str) -> Optional[PostTrainingJobArtifactsResponse]: ...
+    async def get_training_job_artifacts(self, job_uuid: str) -> PostTrainingJobArtifactsResponse: ...

llama_stack/apis/scoring_functions/scoring_functions.py

@@ -135,7 +135,7 @@ class ScoringFunctions(Protocol):
     async def list_scoring_functions(self) -> ListScoringFunctionsResponse: ...
 
     @webmethod(route="/scoring-functions/{scoring_fn_id:path}", method="GET")
-    async def get_scoring_function(self, scoring_fn_id: str, /) -> Optional[ScoringFn]: ...
+    async def get_scoring_function(self, scoring_fn_id: str, /) -> ScoringFn: ...
 
     @webmethod(route="/scoring-functions", method="POST")
     async def register_scoring_function(

llama_stack/apis/shields/shields.py

@@ -49,7 +49,7 @@ class Shields(Protocol):
     async def list_shields(self) -> ListShieldsResponse: ...
 
     @webmethod(route="/shields/{identifier:path}", method="GET")
-    async def get_shield(self, identifier: str) -> Optional[Shield]: ...
+    async def get_shield(self, identifier: str) -> Shield: ...
 
     @webmethod(route="/shields", method="POST")
     async def register_shield(

llama_stack/apis/vector_dbs/vector_dbs.py

@@ -50,7 +50,7 @@ class VectorDBs(Protocol):
     async def get_vector_db(
         self,
         vector_db_id: str,
-    ) -> Optional[VectorDB]: ...
+    ) -> VectorDB: ...
 
     @webmethod(route="/vector-dbs", method="POST")
     async def register_vector_db(