feat: function tools in OpenAI Responses (#2094)

# What does this PR do?

This is a combination of what was previously 3 separate PRs - #2069,
#2075, and #2083. It turns out all 3 of those are needed to land a
working function calling Responses implementation. The web search
builtin tool was already working, but this wires in support for custom
function calling.

I ended up combining all three into one PR because they all had lots of
merge conflicts, both with each other but also with #1806 that just
landed. And, because landing any of them individually would have only
left a partially working implementation merged.

The new things added here are:
* Storing of input items from previous responses and restoring of those
input items when adding previous responses to the conversation state
* Handling of multiple input item messages roles, not just "user"
messages.
* Support for custom tools passed into the Responses API to enable
function calling outside of just the builtin websearch tool.

Closes #2074
Closes #2080

## Test Plan

### Unit Tests

Several new unit tests were added, and they all pass. Ran via:

```
python -m pytest -s -v tests/unit/providers/agents/meta_reference/test_openai_responses.py
```

### Responses API Verification Tests

I ran our verification run.yaml against multiple providers to ensure we
were getting a decent pass rate. Specifically, I ensured the new custom
tool verification test passed across multiple providers and that the
multi-turn examples passed across at least some of the providers (some
providers struggle with the multi-turn workflows still).

Running the stack setup for verification testing:

```
llama stack run --image-type venv tests/verifications/openai-api-verification-run.yaml
```

Together, passing 100% as an example:

```
pytest -s -v 'tests/verifications/openai_api/test_responses.py' --provider=together-llama-stack
```

## Documentation

We will need to start documenting the OpenAI APIs, but for now the
Responses stuff is still rapidly evolving so delaying that.

---------

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

tests/unit/providers/agents/meta_reference/fixtures/__init__.py

@@ -0,0 +1,23 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import os
+
+import yaml
+
+from llama_stack.apis.inference.inference import (
+    OpenAIChatCompletion,
+)
+
+FIXTURES_DIR = os.path.dirname(os.path.abspath(__file__))
+
+
+def load_chat_completion_fixture(filename: str) -> OpenAIChatCompletion:
+    fixture_path = os.path.join(FIXTURES_DIR, filename)
+
+    with open(fixture_path) as f:
+        data = yaml.safe_load(f)
+    return OpenAIChatCompletion(**data)

tests/unit/providers/agents/meta_reference/fixtures/simple_chat_completion.yaml

@@ -0,0 +1,9 @@
+id: chat-completion-123
+choices:
+  - message:
+      content: "Dublin"
+      role: assistant
+    finish_reason: stop
+    index: 0
+created: 1234567890
+model: meta-llama/Llama-3.1-8B-Instruct

tests/unit/providers/agents/meta_reference/fixtures/tool_call_completion.yaml

@@ -0,0 +1,14 @@
+id: chat-completion-123
+choices:
+  - message:
+      tool_calls:
+        - id: tool_call_123
+          type: function
+          function:
+            name: web_search
+            arguments: '{"query":"What is the capital of Ireland?"}'
+      role: assistant
+    finish_reason: stop
+    index: 0
+created: 1234567890
+model: meta-llama/Llama-3.1-8B-Instruct