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/vllm/config.py

@@ -4,7 +4,6 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-from typing import Optional
 
 from pydantic import BaseModel, Field
 
@@ -13,7 +12,7 @@ from llama_stack.schema_utils import json_schema_type
 
 @json_schema_type
 class VLLMInferenceAdapterConfig(BaseModel):
-    url: Optional[str] = Field(
+    url: str | None = Field(
         default=None,
         description="The URL for the vLLM model serving endpoint",
     )
@@ -21,7 +20,7 @@ class VLLMInferenceAdapterConfig(BaseModel):
         default=4096,
         description="Maximum number of tokens to generate.",
     )
-    api_token: Optional[str] = Field(
+    api_token: str | None = Field(
         default="fake",
         description="The API token",
     )