fix: return llama stack model id from embeddings (#3525)

# What does this PR do?

the openai_embeddings method on OpenAIMixin was returning the provider's
model id instead of the llama stack name

## Test Plan

before -
```
$ ./scripts/integration-tests.sh --stack-config server:ci-tests --setup gpt --subdirs inference --inference-mode live --pattern test_openai_embeddings_single_string
...
FAILED tests/integration/inference/test_openai_embeddings.py::test_openai_embeddings_single_string[openai_client-emb=openai/text-embedding-3-small] - AssertionError: assert 'text-embedding-3-small' == 'openai/text-...dding-3-small'
FAILED tests/integration/inference/test_openai_embeddings.py::test_openai_embeddings_single_string[llama_stack_client-emb=openai/text-embedding-3-small] - AssertionError: assert 'text-embedding-3-small' == 'openai/text-...dding-3-small'
========================================== 2 failed, 95 deselected, 4 warnings in 3.87s ===========================================
```
after -
```
$ ./scripts/integration-tests.sh --stack-config server:ci-tests --setup gpt --subdirs inference --inference-mode live --pattern test_openai_embeddings_single_string ...
========================================== 2 passed, 95 deselected, 4 warnings in 2.12s ===========================================
```

Author: mattf

llama_stack/providers/utils/inference/openai_mixin.py

@@ -296,7 +296,7 @@ async def openai_embeddings(
 
         return OpenAIEmbeddingsResponse(
             data=data,
-            model=response.model,
+            model=model,
             usage=usage,
         )
 

tests/integration/suites.py

@@ -87,7 +87,7 @@ class Setup(BaseModel):
         description="OpenAI GPT models for high-quality responses and tool calling",
         defaults={
             "text_model": "openai/gpt-4o",
-            "embedding_model": "sentence-transformers/all-MiniLM-L6-v2",
+            "embedding_model": "openai/text-embedding-3-small",
         },
     ),
     "tgi": Setup(