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>

uv.lock

@@ -410,6 +410,11 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/83/8a/d14e63701c4e869f1d37ba5657f9821961616b98a30074f20b559c071fb6/boto3_stubs-1.40.60-py3-none-any.whl", hash = "sha256:1ea7f9dbabc7f9ac8477646c12cc51ef49af6b24d53cc2ae8cf6fa6bed6a995a", size = 69746, upload-time = "2025-10-27T19:49:05.619Z" },
 ]
 
+[package.optional-dependencies]
+s3 = [
+    { name = "mypy-boto3-s3" },
+]
+
 [[package]]
 name = "botocore"
 version = "1.40.12"
@@ -1871,7 +1876,7 @@ codegen = [
 ]
 dev = [
     { name = "black" },
-    { name = "boto3-stubs" },
+    { name = "boto3-stubs", extra = ["s3"] },
     { name = "mypy" },
     { name = "nbval" },
     { name = "pandas-stubs" },
@@ -1995,7 +2000,7 @@ codegen = [
 ]
 dev = [
     { name = "black" },
-    { name = "boto3-stubs" },
+    { name = "boto3-stubs", extras = ["s3"] },
     { name = "mypy" },
     { name = "nbval" },
     { name = "pandas-stubs" },
@@ -2568,6 +2573,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/87/e3/be76d87158ebafa0309946c4a73831974d4d6ab4f4ef40c3b53a385a66fd/mypy-1.18.2-py3-none-any.whl", hash = "sha256:22a1748707dd62b58d2ae53562ffc4d7f8bcc727e8ac7cbc69c053ddc874d47e", size = 2352367, upload-time = "2025-09-19T00:10:15.489Z" },
 ]
 
+[[package]]
+name = "mypy-boto3-s3"
+version = "1.40.26"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/00/b8/55d21ed9ca479df66d9892212ba7d7977850ef17aa80a83e3f11f31190fd/mypy_boto3_s3-1.40.26.tar.gz", hash = "sha256:8d2bfd1052894d0e84c9fb9358d838ba0eed0265076c7dd7f45622c770275c99", size = 75948, upload-time = "2025-09-08T20:12:21.405Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/85/a5/dba3384423834009bdd41c7021de5c663468a0e7bc4071cb301721e52a99/mypy_boto3_s3-1.40.26-py3-none-any.whl", hash = "sha256:6d055d16ef89a0133ade92f6b4f09603e4acc31a0f5e8f846edf4eb48f17b5a7", size = 82762, upload-time = "2025-09-08T20:12:19.338Z" },
+]
+
 [[package]]
 name = "mypy-extensions"
 version = "1.1.0"