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/distribution/providers.py

@@ -5,7 +5,7 @@
 # the root directory of this source tree.
 
 import asyncio
-from typing import Any, Dict
+from typing import Any
 
 from pydantic import BaseModel
 
@@ -73,14 +73,14 @@ class ProviderImpl(Providers):
 
         raise ValueError(f"Provider {provider_id} not found")
 
-    async def get_providers_health(self) -> Dict[str, Dict[str, HealthResponse]]:
+    async def get_providers_health(self) -> dict[str, dict[str, HealthResponse]]:
         """Get health status for all providers.
 
         Returns:
             Dict[str, Dict[str, HealthResponse]]: A dictionary mapping API names to provider health statuses.
                 Each API maps to a dictionary of provider IDs to their health responses.
         """
-        providers_health: Dict[str, Dict[str, HealthResponse]] = {}
+        providers_health: dict[str, dict[str, HealthResponse]] = {}
         timeout = 1.0
 
         async def check_provider_health(impl: Any) -> tuple[str, HealthResponse] | None: