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/fixtures/fixtures.py

@@ -5,7 +5,6 @@
 # the root directory of this source tree.
 
 import os
-import re
 from pathlib import Path
 
 import pytest
@@ -48,19 +47,6 @@ def _load_all_verification_configs():
     return {"providers": all_provider_configs}
 
 
-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
-
-
-# Helper to get the base test name from the request object
-def get_base_test_name(request):
-    return request.node.originalname
-
-
 # --- End Helper Functions ---