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/datasetio/nvidia/config.py

@@ -6,7 +6,7 @@
 
 import os
 import warnings
-from typing import Any, Dict, Optional
+from typing import Any
 
 from pydantic import BaseModel, Field
 
@@ -14,17 +14,17 @@ from pydantic import BaseModel, Field
 class NvidiaDatasetIOConfig(BaseModel):
     """Configuration for NVIDIA DatasetIO implementation."""
 
-    api_key: Optional[str] = Field(
+    api_key: str | None = Field(
         default_factory=lambda: os.getenv("NVIDIA_API_KEY"),
         description="The NVIDIA API key.",
     )
 
-    dataset_namespace: Optional[str] = Field(
+    dataset_namespace: str | None = Field(
         default_factory=lambda: os.getenv("NVIDIA_DATASET_NAMESPACE", "default"),
         description="The NVIDIA dataset namespace.",
     )
 
-    project_id: Optional[str] = Field(
+    project_id: str | None = Field(
         default_factory=lambda: os.getenv("NVIDIA_PROJECT_ID", "test-project"),
         description="The NVIDIA project ID.",
     )
@@ -52,7 +52,7 @@ class NvidiaDatasetIOConfig(BaseModel):
             )
 
     @classmethod
-    def sample_run_config(cls, **kwargs) -> Dict[str, Any]:
+    def sample_run_config(cls, **kwargs) -> dict[str, Any]:
         return {
             "api_key": "${env.NVIDIA_API_KEY:}",
             "dataset_namespace": "${env.NVIDIA_DATASET_NAMESPACE:default}",