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/safety/nvidia/nvidia.py

@@ -5,7 +5,7 @@
 # the root directory of this source tree.
 
 import logging
-from typing import Any, List, Optional
+from typing import Any
 
 import requests
 
@@ -41,7 +41,7 @@ class NVIDIASafetyAdapter(Safety, ShieldsProtocolPrivate):
             raise ValueError("Shield model not provided.")
 
     async def run_shield(
-        self, shield_id: str, messages: List[Message], params: Optional[dict[str, Any]] = None
+        self, shield_id: str, messages: list[Message], params: dict[str, Any] | None = None
     ) -> RunShieldResponse:
         """
         Run a safety shield check against the provided messages.
@@ -112,7 +112,7 @@ class NeMoGuardrails:
         response.raise_for_status()
         return response.json()
 
-    async def run(self, messages: List[Message]) -> RunShieldResponse:
+    async def run(self, messages: list[Message]) -> RunShieldResponse:
         """
         Queries the /v1/guardrails/checks endpoint of the NeMo guardrails deployed API.