More work on file_search verification test

This gets the file_search verification test working against ollama,
fireworks, and api.openai.com. We don't have the entirety of the
vector store API implemented in Llama Stack yet, so this still has a
bit of a hack to swap between using only OpenAI-compatible APIs versus
using the LlamaStackClient to insert content into our vector stores.

Outside of actually inserting file contents, the rest of the test
works the same and uses only the OpenAI client for all of these providers.

How to run the tests:

Ollama (sometimes flakes with small model):

```
ollama run llama3.2:3b

INFERENCE_MODEL="meta-llama/Llama-3.2-3B-Instruct" \
llama stack run ./llama_stack/templates/ollama/run.yaml \
  --image-type venv \
  --env OLLAMA_URL="http://0.0.0.0:11434"

pytest -sv \
  'tests/verifications/openai_api/test_responses.py::test_response_non_streaming_file_search' \
  --base-url=http://localhost:8321/v1/openai/v1 \
  --model meta-llama/Llama-3.2-3B-Instruct
```

Fireworks via Llama Stack:

```
llama stack run llama_stack/templates/fireworks/run.yaml

pytest -sv \
  'tests/verifications/openai_api/test_responses.py::test_response_non_streaming_file_search' \
  --base-url=http://localhost:8321/v1/openai/v1 \
  --model meta-llama/Llama-3.3-70B-Instruct
```

OpenAI directly:

```
pytest -sv \
  'tests/verifications/openai_api/test_responses.py::test_response_non_streaming_file_search' \
  --base-url=https://api.openai.com/v1 \
  --model gpt-4o
```

Signed-off-by: Ben Browning <bbrownin@redhat.com>

tests/verifications/openai_api/fixtures/test_cases/responses.yaml

@@ -39,8 +39,7 @@ test_response_file_search:
       input: "How many experts does the Llama 4 Maverick model have?"
       tools:
       - type: file_search
-        vector_store_ids:
-        - test_vector_store
+        # vector_store_ids gets added by the test runner
       output: "128"
 
 test_response_mcp_tool: