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>

pyproject.toml

@@ -67,8 +67,8 @@ dev = [
     "pytest-cov",
     "pytest-html",
     "pytest-json-report",
-    "pytest-socket",       # For blocking network access in unit tests
-    "nbval",               # For notebook testing
+    "pytest-socket", # For blocking network access in unit tests
+    "nbval", # For notebook testing
     "black",
     "ruff",
     "mypy",
@@ -78,9 +78,9 @@ dev = [
     "pandas-stubs",
     "types-psutil",
     "types-tqdm",
-    "boto3-stubs",
+    "boto3-stubs[s3]",
     "pre-commit",
-    "ruamel.yaml",         # needed for openapi generator
+    "ruamel.yaml", # needed for openapi generator
 ]
 # These are the dependencies required for running unit tests.
 unit = [