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>

docs/openapi_generator/generate.py

@@ -12,7 +12,7 @@
 
 from datetime import datetime
 from pathlib import Path
-
+import sys
 import fire
 import ruamel.yaml as yaml
 
@@ -21,7 +21,7 @@ from llama_stack.distribution.stack import LlamaStack  # noqa: E402
 
 from .pyopenapi.options import Options  # noqa: E402
 from .pyopenapi.specification import Info, Server  # noqa: E402
-from .pyopenapi.utility import Specification  # noqa: E402
+from .pyopenapi.utility import Specification, validate_api_method_return_types  # noqa: E402
 
 
 def str_presenter(dumper, data):
@@ -39,6 +39,14 @@ def main(output_dir: str):
     if not output_dir.exists():
         raise ValueError(f"Directory {output_dir} does not exist")
 
+    # Validate API protocols before generating spec
+    print("Validating API method return types...")
+    return_type_errors = validate_api_method_return_types()
+    if return_type_errors:
+        print("\nAPI Method Return Type Validation Errors:\n")
+        for error in return_type_errors:
+            print(error)
+        sys.exit(1)
     now = str(datetime.now())
     print(
         "Converting the spec to YAML (openapi.yaml) and HTML (openapi.html) at " + now