refactor(tests): make the responses tests nicer (#3161)

# What does this PR do?

A _bunch_ on cleanup for the Responses tests.

- Got rid of YAML test cases, moved them to just use simple pydantic models
- Splitting the large monolithic test file into multiple focused test files:
   - `test_basic_responses.py` for basic and image response tests
   - `test_tool_responses.py` for tool-related tests
   - `test_file_search.py` for file search specific tests
- Adding a `StreamingValidator` helper class to standardize streaming response validation

## Test Plan

Run the tests:

```
pytest -s -v tests/integration/non_ci/responses/ \
   --stack-config=starter \
   --text-model openai/gpt-4o \
   --embedding-model=sentence-transformers/all-MiniLM-L6-v2 \
    -k "client_with_models"
```

tests/integration/non_ci/responses/streaming_assertions.py

@@ -0,0 +1,145 @@
+# 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.
+
+from typing import Any
+
+
+class StreamingValidator:
+    """Helper class for validating streaming response events."""
+
+    def __init__(self, chunks: list[Any]):
+        self.chunks = chunks
+        self.event_types = [chunk.type for chunk in chunks]
+
+    def assert_basic_event_sequence(self):
+        """Verify basic created -> completed event sequence."""
+        assert len(self.chunks) >= 2, f"Expected at least 2 chunks (created + completed), got {len(self.chunks)}"
+        assert self.chunks[0].type == "response.created", (
+            f"First chunk should be response.created, got {self.chunks[0].type}"
+        )
+        assert self.chunks[-1].type == "response.completed", (
+            f"Last chunk should be response.completed, got {self.chunks[-1].type}"
+        )
+
+        # Verify event order
+        created_index = self.event_types.index("response.created")
+        completed_index = self.event_types.index("response.completed")
+        assert created_index < completed_index, "response.created should come before response.completed"
+
+    def assert_response_consistency(self):
+        """Verify response ID consistency across events."""
+        response_ids = set()
+        for chunk in self.chunks:
+            if hasattr(chunk, "response_id"):
+                response_ids.add(chunk.response_id)
+            elif hasattr(chunk, "response") and hasattr(chunk.response, "id"):
+                response_ids.add(chunk.response.id)
+
+        assert len(response_ids) == 1, f"All events should reference the same response_id, found: {response_ids}"
+
+    def assert_has_incremental_content(self):
+        """Verify that content is delivered incrementally via delta events."""
+        delta_events = [
+            i for i, event_type in enumerate(self.event_types) if event_type == "response.output_text.delta"
+        ]
+        assert len(delta_events) > 0, "Expected delta events for true incremental streaming, but found none"
+
+        # Verify delta events have content
+        non_empty_deltas = 0
+        delta_content_total = ""
+
+        for delta_idx in delta_events:
+            chunk = self.chunks[delta_idx]
+            if hasattr(chunk, "delta") and chunk.delta:
+                delta_content_total += chunk.delta
+                non_empty_deltas += 1
+
+        assert non_empty_deltas > 0, "Delta events found but none contain content"
+        assert len(delta_content_total) > 0, "Delta events found but total delta content is empty"
+
+        return delta_content_total
+
+    def assert_content_quality(self, expected_content: str):
+        """Verify the final response contains expected content."""
+        final_chunk = self.chunks[-1]
+        if hasattr(final_chunk, "response"):
+            output_text = final_chunk.response.output_text.lower().strip()
+            assert len(output_text) > 0, "Response should have content"
+            assert expected_content.lower() in output_text, f"Expected '{expected_content}' in response"
+
+    def assert_has_tool_calls(self):
+        """Verify tool call streaming events are present."""
+        # Check for tool call events
+        delta_events = [
+            chunk
+            for chunk in self.chunks
+            if chunk.type in ["response.function_call_arguments.delta", "response.mcp_call.arguments.delta"]
+        ]
+        done_events = [
+            chunk
+            for chunk in self.chunks
+            if chunk.type in ["response.function_call_arguments.done", "response.mcp_call.arguments.done"]
+        ]
+
+        assert len(delta_events) > 0, f"Expected tool call delta events, got chunk types: {self.event_types}"
+        assert len(done_events) > 0, f"Expected tool call done events, got chunk types: {self.event_types}"
+
+        # Verify output item events
+        item_added_events = [chunk for chunk in self.chunks if chunk.type == "response.output_item.added"]
+        item_done_events = [chunk for chunk in self.chunks if chunk.type == "response.output_item.done"]
+
+        assert len(item_added_events) > 0, (
+            f"Expected response.output_item.added events, got chunk types: {self.event_types}"
+        )
+        assert len(item_done_events) > 0, (
+            f"Expected response.output_item.done events, got chunk types: {self.event_types}"
+        )
+
+    def assert_has_mcp_events(self):
+        """Verify MCP-specific streaming events are present."""
+        # Tool execution progress events
+        mcp_in_progress_events = [chunk for chunk in self.chunks if chunk.type == "response.mcp_call.in_progress"]
+        mcp_completed_events = [chunk for chunk in self.chunks if chunk.type == "response.mcp_call.completed"]
+
+        assert len(mcp_in_progress_events) > 0, (
+            f"Expected response.mcp_call.in_progress events, got chunk types: {self.event_types}"
+        )
+        assert len(mcp_completed_events) > 0, (
+            f"Expected response.mcp_call.completed events, got chunk types: {self.event_types}"
+        )
+
+        # MCP list tools events
+        mcp_list_tools_in_progress_events = [
+            chunk for chunk in self.chunks if chunk.type == "response.mcp_list_tools.in_progress"
+        ]
+        mcp_list_tools_completed_events = [
+            chunk for chunk in self.chunks if chunk.type == "response.mcp_list_tools.completed"
+        ]
+
+        assert len(mcp_list_tools_in_progress_events) > 0, (
+            f"Expected response.mcp_list_tools.in_progress events, got chunk types: {self.event_types}"
+        )
+        assert len(mcp_list_tools_completed_events) > 0, (
+            f"Expected response.mcp_list_tools.completed events, got chunk types: {self.event_types}"
+        )
+
+    def assert_rich_streaming(self, min_chunks: int = 10):
+        """Verify we have substantial streaming activity."""
+        assert len(self.chunks) > min_chunks, (
+            f"Expected rich streaming with many events, got only {len(self.chunks)} chunks"
+        )
+
+    def validate_event_structure(self):
+        """Validate the structure of various event types."""
+        for chunk in self.chunks:
+            if chunk.type == "response.created":
+                assert chunk.response.status == "in_progress"
+            elif chunk.type == "response.completed":
+                assert chunk.response.status == "completed"
+            elif hasattr(chunk, "item_id"):
+                assert chunk.item_id, "Events with item_id should have non-empty item_id"
+            elif hasattr(chunk, "sequence_number"):
+                assert isinstance(chunk.sequence_number, int), "sequence_number should be an integer"