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/distribution/routers/routing_tables.py

@@ -219,8 +219,11 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
     async def list_models(self) -> ListModelsResponse:
         return ListModelsResponse(data=await self.get_all_with_type("model"))
 
-    async def get_model(self, model_id: str) -> Optional[Model]:
-        return await self.get_object_by_identifier("model", model_id)
+    async def get_model(self, model_id: str) -> Model:
+        model = await self.get_object_by_identifier("model", model_id)
+        if model is None:
+            raise ValueError(f"Model '{model_id}' not found")
+        return model
 
     async def register_model(
         self,
@@ -267,8 +270,11 @@ class ShieldsRoutingTable(CommonRoutingTableImpl, Shields):
     async def list_shields(self) -> ListShieldsResponse:
         return ListShieldsResponse(data=await self.get_all_with_type(ResourceType.shield.value))
 
-    async def get_shield(self, identifier: str) -> Optional[Shield]:
-        return await self.get_object_by_identifier("shield", identifier)
+    async def get_shield(self, identifier: str) -> Shield:
+        shield = await self.get_object_by_identifier("shield", identifier)
+        if shield is None:
+            raise ValueError(f"Shield '{identifier}' not found")
+        return shield
 
     async def register_shield(
         self,
@@ -303,8 +309,11 @@ class VectorDBsRoutingTable(CommonRoutingTableImpl, VectorDBs):
     async def list_vector_dbs(self) -> ListVectorDBsResponse:
         return ListVectorDBsResponse(data=await self.get_all_with_type("vector_db"))
 
-    async def get_vector_db(self, vector_db_id: str) -> Optional[VectorDB]:
-        return await self.get_object_by_identifier("vector_db", vector_db_id)
+    async def get_vector_db(self, vector_db_id: str) -> VectorDB:
+        vector_db = await self.get_object_by_identifier("vector_db", vector_db_id)
+        if vector_db is None:
+            raise ValueError(f"Vector DB '{vector_db_id}' not found")
+        return vector_db
 
     async def register_vector_db(
         self,
@@ -355,8 +364,11 @@ class DatasetsRoutingTable(CommonRoutingTableImpl, Datasets):
     async def list_datasets(self) -> ListDatasetsResponse:
         return ListDatasetsResponse(data=await self.get_all_with_type(ResourceType.dataset.value))
 
-    async def get_dataset(self, dataset_id: str) -> Optional[Dataset]:
-        return await self.get_object_by_identifier("dataset", dataset_id)
+    async def get_dataset(self, dataset_id: str) -> Dataset:
+        dataset = await self.get_object_by_identifier("dataset", dataset_id)
+        if dataset is None:
+            raise ValueError(f"Dataset '{dataset_id}' not found")
+        return dataset
 
     async def register_dataset(
         self,
@@ -408,8 +420,11 @@ class ScoringFunctionsRoutingTable(CommonRoutingTableImpl, ScoringFunctions):
     async def list_scoring_functions(self) -> ListScoringFunctionsResponse:
         return ListScoringFunctionsResponse(data=await self.get_all_with_type(ResourceType.scoring_function.value))
 
-    async def get_scoring_function(self, scoring_fn_id: str) -> Optional[ScoringFn]:
-        return await self.get_object_by_identifier("scoring_function", scoring_fn_id)
+    async def get_scoring_function(self, scoring_fn_id: str) -> ScoringFn:
+        scoring_fn = await self.get_object_by_identifier("scoring_function", scoring_fn_id)
+        if scoring_fn is None:
+            raise ValueError(f"Scoring function '{scoring_fn_id}' not found")
+        return scoring_fn
 
     async def register_scoring_function(
         self,
@@ -445,8 +460,11 @@ class BenchmarksRoutingTable(CommonRoutingTableImpl, Benchmarks):
     async def list_benchmarks(self) -> ListBenchmarksResponse:
         return ListBenchmarksResponse(data=await self.get_all_with_type("benchmark"))
 
-    async def get_benchmark(self, benchmark_id: str) -> Optional[Benchmark]:
-        return await self.get_object_by_identifier("benchmark", benchmark_id)
+    async def get_benchmark(self, benchmark_id: str) -> Benchmark:
+        benchmark = await self.get_object_by_identifier("benchmark", benchmark_id)
+        if benchmark is None:
+            raise ValueError(f"Benchmark '{benchmark_id}' not found")
+        return benchmark
 
     async def register_benchmark(
         self,
@@ -490,7 +508,10 @@ class ToolGroupsRoutingTable(CommonRoutingTableImpl, ToolGroups):
         return ListToolGroupsResponse(data=await self.get_all_with_type("tool_group"))
 
     async def get_tool_group(self, toolgroup_id: str) -> ToolGroup:
-        return await self.get_object_by_identifier("tool_group", toolgroup_id)
+        tool_group = await self.get_object_by_identifier("tool_group", toolgroup_id)
+        if tool_group is None:
+            raise ValueError(f"Tool group '{toolgroup_id}' not found")
+        return tool_group
 
     async def get_tool(self, tool_name: str) -> Tool:
         return await self.get_object_by_identifier("tool", tool_name)