chore: enable pyupgrade fixes (#1806)

# What does this PR do?

The goal of this PR is code base modernization.

Schema reflection code needed a minor adjustment to handle UnionTypes
and collections.abc.AsyncIterator. (Both are preferred for latest Python
releases.)

Note to reviewers: almost all changes here are automatically generated
by pyupgrade. Some additional unused imports were cleaned up. The only
change worth of note can be found under `docs/openapi_generator` and
`llama_stack/strong_typing/schema.py` where reflection code was updated
to deal with "newer" types.

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

llama_stack/providers/remote/inference/nvidia/utils.py

@@ -5,7 +5,6 @@
 # the root directory of this source tree.
 
 import logging
-from typing import Tuple
 
 import httpx
 
@@ -18,7 +17,7 @@ def _is_nvidia_hosted(config: NVIDIAConfig) -> bool:
     return "integrate.api.nvidia.com" in config.url
 
 
-async def _get_health(url: str) -> Tuple[bool, bool]:
+async def _get_health(url: str) -> tuple[bool, bool]:
     """
     Query {url}/v1/health/{live,ready} to check if the server is running and ready