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-verification-run.yaml

@@ -1,10 +1,15 @@
+# This is a temporary run file because model names used by the verification tests
+# are not quite consistent with various pre-existing distributions.
+#
 version: '2'
 image_name: openai-api-verification
 apis:
+- agents
 - inference
 - telemetry
 - tool_runtime
 - vector_io
+- safety
 providers:
   inference:
   - provider_id: together
@@ -16,12 +21,12 @@ providers:
     provider_type: remote::fireworks
     config:
       url: https://api.fireworks.ai/inference/v1
-      api_key: ${env.FIREWORKS_API_KEY}
+      api_key: ${env.FIREWORKS_API_KEY:}
   - provider_id: groq
     provider_type: remote::groq
     config:
       url: https://api.groq.com
-      api_key: ${env.GROQ_API_KEY}
+      api_key: ${env.GROQ_API_KEY:}
   - provider_id: openai
     provider_type: remote::openai
     config:
@@ -45,6 +50,19 @@ providers:
       service_name: "${env.OTEL_SERVICE_NAME:\u200B}"
       sinks: ${env.TELEMETRY_SINKS:console,sqlite}
       sqlite_db_path: ${env.SQLITE_DB_PATH:~/.llama/distributions/openai/trace_store.db}
+  safety:
+  - provider_id: llama-guard
+    provider_type: inline::llama-guard
+    config:
+      excluded_categories: []
+  agents:
+  - provider_id: meta-reference
+    provider_type: inline::meta-reference
+    config:
+      persistence_store:
+        type: sqlite
+        namespace: null
+        db_path: ${env.SQLITE_STORE_DIR:~/.llama/distributions/openai}/agents_store.db
   tool_runtime:
   - provider_id: brave-search
     provider_type: remote::brave-search