chore: Don't hide stderr from api generator (#1720)

# What does this PR do?

If the generator fails, pre-commit logs will now show how it failed.

Note: stdout is still suppressed, so that regular informational messages
do not pollute pre-commit output when all the hook does is update
generated files.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan

Inject a failure in the generator code and confirm it's seen in the
output.

```
$ git diff
diff --git a/docs/openapi_generator/pyopenapi/utility.py b/docs/openapi_generator/pyopenapi/utility.py
index f60a33bb..482e26ef 100644
--- a/docs/openapi_generator/pyopenapi/utility.py
+++ b/docs/openapi_generator/pyopenapi/utility.py
@@ -127,6 +127,7 @@ def is_optional_type(type_: Any) -> bool:

 def validate_api_method_return_types() -> List[str]:
     """Validate that all API methods have proper return types."""
+    raise NotImplementedError("This function is not implemented yet")
     errors = []
     protocols = api_protocol_map()
```

```
$ pre-commit run --all-files
check for merge conflicts................................................Passed
trim trailing whitespace.................................................Passed
check for added large files..............................................Passed
fix end of files.........................................................Passed
Insert license in comments...............................................Passed
ruff.....................................................................Passed
ruff-format..............................................................Passed
blacken-docs.............................................................Passed
uv-lock..................................................................Passed
uv-export................................................................Passed
mypy.....................................................................Passed
Distribution Template Codegen............................................Passed
API Spec Codegen.........................................................Failed
- hook id: openapi-codegen
- exit code: 1

warning: `VIRTUAL_ENV=/Users/ihrachys/.cache/pre-commit/repo9p35zuhm/py_env-python3` does not match the project environment path `.venv` and will be ignored; use `--active` to target the active environment instead
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/generate.py", line 91, in <module>
    fire.Fire(main)
  File "/Users/ihrachys/.cache/uv/archive-v0/FBgkcwcN-PaJ0NAur__7J/lib/python3.11/site-packages/fire/core.py", line 135, in Fire
    component_trace = _Fire(component, args, parsed_flag_args, context, name)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/.cache/uv/archive-v0/FBgkcwcN-PaJ0NAur__7J/lib/python3.11/site-packages/fire/core.py", line 468, in _Fire
    component, remaining_args = _CallAndUpdateTrace(
                                ^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/.cache/uv/archive-v0/FBgkcwcN-PaJ0NAur__7J/lib/python3.11/site-packages/fire/core.py", line 684, in _CallAndUpdateTrace
    component = fn(*varargs, **kwargs)
                ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/generate.py", line 44, in main
    return_type_errors = validate_api_method_return_types()
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/pyopenapi/utility.py", line 130, in validate_api_method_return_types
    raise NotImplementedError("This function is not implemented yet")
NotImplementedError: This function is not implemented yet
```

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

.pre-commit-config.yaml

@@ -89,7 +89,7 @@ repos:
         name: API Spec Codegen
         additional_dependencies:
           - uv==0.6.2
-        entry: sh -c 'uv run --with ".[dev]" ./docs/openapi_generator/run_openapi_generator.sh > /dev/null 2>&1'
+        entry: sh -c 'uv run --with ".[dev]" ./docs/openapi_generator/run_openapi_generator.sh > /dev/null'
         language: python
         pass_filenames: false
         require_serial: true