Fix OpenAI structured chat response handling

[Kevin-Li-2025]

Summary

• replace the undefined handle_response call in OpenAIModelAdapter.structured_chat with the existing response handling pattern used by chat
• add a focused unit test that uses a fake Azure client and does not require live credentials

Fixes #36.

Validation

• uv run --python 3.12 --with pytest --with pytest-asyncio --with pydantic --with pydantic-settings --with python-dotenv --with i-dot-ai-utilities --with openai --with google-genai pytest tests/test_azure_openai_adapter.py -q
• uv run --python 3.12 --with ruff ruff check common/llm/adapters/azure_openai.py tests/test_azure_openai_adapter.py
• uv run --python 3.12 --with pydantic --with pydantic-settings --with python-dotenv --with i-dot-ai-utilities --with openai --with google-genai python -m py_compile common/llm/adapters/azure_openai.py tests/test_azure_openai_adapter.py