feat(verification): various improvements (#1921)

# What does this PR do?
- provider and their models now live in config.yaml
- better distinguish different cases within a test
- add model key to surface provider's model_id
- include example command to rerun single test case

## Test Plan
<img width="1173" alt="image"
src="https://github.com/user-attachments/assets/b414baf0-c768-451f-8c3b-c2905cf36fac"
/>

tests/verifications/openai_api/fixtures/fixtures.py

@@ -0,0 +1,105 @@
+# 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
+from pathlib import Path
+
+import pytest
+import yaml
+from openai import OpenAI
+
+
+# --- Helper Function to Load Config ---
+def _load_all_verification_configs():
+    """Load and aggregate verification configs from the conf/ directory."""
+    # Note: Path is relative to *this* file (fixtures.py)
+    conf_dir = Path(__file__).parent.parent.parent / "conf"
+    if not conf_dir.is_dir():
+        # Use pytest.fail if called during test collection, otherwise raise error
+        # For simplicity here, we'll raise an error, assuming direct calls
+        # are less likely or can handle it.
+        raise FileNotFoundError(f"Verification config directory not found at {conf_dir}")
+
+    all_provider_configs = {}
+    yaml_files = list(conf_dir.glob("*.yaml"))
+    if not yaml_files:
+        raise FileNotFoundError(f"No YAML configuration files found in {conf_dir}")
+
+    for config_path in yaml_files:
+        provider_name = config_path.stem
+        try:
+            with open(config_path, "r") as f:
+                provider_config = yaml.safe_load(f)
+                if provider_config:
+                    all_provider_configs[provider_name] = provider_config
+                else:
+                    # Log warning if possible, or just skip empty files silently
+                    print(f"Warning: Config file {config_path} is empty or invalid.")
+        except Exception as e:
+            raise IOError(f"Error loading config file {config_path}: {e}") from e
+
+    return {"providers": all_provider_configs}
+
+
+# --- End Helper Function ---
+
+
+@pytest.fixture(scope="session")
+def verification_config():
+    """Pytest fixture to provide the loaded verification config."""
+    try:
+        return _load_all_verification_configs()
+    except (FileNotFoundError, IOError) as e:
+        pytest.fail(str(e))  # Fail test collection if config loading fails
+
+
+@pytest.fixture
+def provider(request, verification_config):
+    provider = request.config.getoption("--provider")
+    base_url = request.config.getoption("--base-url")
+
+    if provider and base_url and verification_config["providers"][provider]["base_url"] != base_url:
+        raise ValueError(f"Provider {provider} is not supported for base URL {base_url}")
+
+    if not provider:
+        if not base_url:
+            raise ValueError("Provider and base URL are not provided")
+        for provider, metadata in verification_config["providers"].items():
+            if metadata["base_url"] == base_url:
+                provider = provider
+                break
+
+    return provider
+
+
+@pytest.fixture
+def base_url(request, provider, verification_config):
+    return request.config.getoption("--base-url") or verification_config["providers"][provider]["base_url"]
+
+
+@pytest.fixture
+def api_key(request, provider, verification_config):
+    provider_conf = verification_config.get("providers", {}).get(provider, {})
+    api_key_env_var = provider_conf.get("api_key_var")
+
+    key_from_option = request.config.getoption("--api-key")
+    key_from_env = os.getenv(api_key_env_var) if api_key_env_var else None
+
+    final_key = key_from_option or key_from_env
+    return final_key
+
+
+@pytest.fixture
+def model_mapping(provider, providers_model_mapping):
+    return providers_model_mapping[provider]
+
+
+@pytest.fixture
+def openai_client(base_url, api_key):
+    return OpenAI(
+        base_url=base_url,
+        api_key=api_key,
+    )