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/helpers.py

@@ -0,0 +1,64 @@
+# 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 time
+
+
+def new_vector_store(openai_client, name):
+    """Create a new vector store, cleaning up any existing one with the same name."""
+    # Ensure we don't reuse an existing vector store
+    vector_stores = openai_client.vector_stores.list()
+    for vector_store in vector_stores:
+        if vector_store.name == name:
+            openai_client.vector_stores.delete(vector_store_id=vector_store.id)
+
+    # Create a new vector store
+    vector_store = openai_client.vector_stores.create(name=name)
+    return vector_store
+
+
+def upload_file(openai_client, name, file_path):
+    """Upload a file, cleaning up any existing file with the same name."""
+    # Ensure we don't reuse an existing file
+    files = openai_client.files.list()
+    for file in files:
+        if file.filename == name:
+            openai_client.files.delete(file_id=file.id)
+
+    # Upload a text file with our document content
+    return openai_client.files.create(file=open(file_path, "rb"), purpose="assistants")
+
+
+def wait_for_file_attachment(compat_client, vector_store_id, file_id):
+    """Wait for a file to be attached to a vector store."""
+    file_attach_response = compat_client.vector_stores.files.retrieve(
+        vector_store_id=vector_store_id,
+        file_id=file_id,
+    )
+
+    while file_attach_response.status == "in_progress":
+        time.sleep(0.1)
+        file_attach_response = compat_client.vector_stores.files.retrieve(
+            vector_store_id=vector_store_id,
+            file_id=file_id,
+        )
+
+    assert file_attach_response.status == "completed", f"Expected file to be attached, got {file_attach_response}"
+    assert not file_attach_response.last_error
+    return file_attach_response
+
+
+def setup_mcp_tools(tools, mcp_server_info):
+    """Replace placeholder MCP server URLs with actual server info."""
+    # Create a deep copy to avoid modifying the original test case
+    import copy
+
+    tools_copy = copy.deepcopy(tools)
+
+    for tool in tools_copy:
+        if tool["type"] == "mcp" and tool["server_url"] == "<FILLED_BY_TEST_RUNNER>":
+            tool["server_url"] = mcp_server_info["server_url"]
+    return tools_copy