fix(mypy): resolve OpenAI SDK and provider type issues (#3936)

## Summary
- Fix OpenAI SDK NotGiven/Omit type mismatches in embeddings calls
- Fix incorrect OpenAIChatCompletionChunk import in vllm provider
- Refactor to avoid type:ignore comments by using conditional kwargs

## Changes
**openai_mixin.py (9 errors fixed):**
- Build kwargs conditionally for embeddings.create() to avoid
NotGiven/Omit mismatch
- Only include parameters when they have actual values (not None)

**gemini.py (9 errors fixed):**
- Apply same conditional kwargs pattern
- Add missing Any import

**vllm.py (2 errors fixed):**
- Use correct OpenAIChatCompletionChunk from llama_stack.apis.inference
- Remove incorrect alias from openai package

## Technical Notes
The OpenAI SDK has a type system quirk where `NOT_GIVEN` has type
`NotGiven` but parameter signatures expect `Omit`. By only passing
parameters with actual values, we avoid this mismatch entirely without
needing `# type: ignore` comments.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/llama_stack/providers/remote/inference/vllm/vllm.py

@@ -7,13 +7,11 @@ from collections.abc import AsyncIterator
 from urllib.parse import urljoin
 
 import httpx
-from openai.types.chat.chat_completion_chunk import (
-    ChatCompletionChunk as OpenAIChatCompletionChunk,
-)
 from pydantic import ConfigDict
 
 from llama_stack.apis.inference import (
     OpenAIChatCompletion,
+    OpenAIChatCompletionChunk,
     OpenAIChatCompletionRequestWithExtraBody,
     ToolChoice,
 )