feat: OpenAI Responses API (#1989)

# What does this PR do?

This provides an initial [OpenAI Responses
API](https://platform.openai.com/docs/api-reference/responses)
implementation. The API is not yet complete, and this is more a
proof-of-concept to show how we can store responses in our key-value
stores and use them to support the Responses API concepts like
`previous_response_id`.

## Test Plan

I've added a new
`tests/integration/openai_responses/test_openai_responses.py` as part of
a test-driven development for this new API. I'm only testing this
locally with the remote-vllm provider for now, but it should work with
any of our inference providers since the only API it requires out of the
inference provider is the `openai_chat_completion` endpoint.

```
VLLM_URL="http://localhost:8000/v1" \
INFERENCE_MODEL="meta-llama/Llama-3.2-3B-Instruct" \
llama stack build --template remote-vllm --image-type venv --run
```

```
LLAMA_STACK_CONFIG="http://localhost:8321" \
python -m pytest -v \
  tests/integration/openai_responses/test_openai_responses.py \
  --text-model "meta-llama/Llama-3.2-3B-Instruct"
 ```

---------

Signed-off-by: Ben Browning <bbrownin@redhat.com>
Co-authored-by: Ashwin Bharambe <ashwin.bharambe@gmail.com>

tests/verifications/openai_api/fixtures/fixtures.py

@@ -5,14 +5,16 @@
 # the root directory of this source tree.
 
 import os
+import re
 from pathlib import Path
 
 import pytest
 import yaml
 from openai import OpenAI
 
+# --- Helper Functions ---
+
 
-# --- Helper Function to Load Config ---
 def _load_all_verification_configs():
     """Load and aggregate verification configs from the conf/ directory."""
     # Note: Path is relative to *this* file (fixtures.py)
@@ -44,7 +46,30 @@ def _load_all_verification_configs():
     return {"providers": all_provider_configs}
 
 
-# --- End Helper Function ---
+def case_id_generator(case):
+    """Generate a test ID from the case's 'case_id' field, or use a default."""
+    case_id = case.get("case_id")
+    if isinstance(case_id, (str, int)):
+        return re.sub(r"\\W|^(?=\\d)", "_", str(case_id))
+    return None
+
+
+def should_skip_test(verification_config, provider, model, test_name_base):
+    """Check if a test should be skipped based on config exclusions."""
+    provider_config = verification_config.get("providers", {}).get(provider)
+    if not provider_config:
+        return False  # No config for provider, don't skip
+
+    exclusions = provider_config.get("test_exclusions", {}).get(model, [])
+    return test_name_base in exclusions
+
+
+# Helper to get the base test name from the request object
+def get_base_test_name(request):
+    return request.node.originalname
+
+
+# --- End Helper Functions ---
 
 
 @pytest.fixture(scope="session")