Migration request: Chat Completions to Responses

Migrate this package from the legacy Chat Completions call shape to the Responses API call shape.

Current structure

• customer_support_bot/client.py contains the OpenAI client wrapper.
• customer_support_bot/replies.py builds the support reply prompt and calls the wrapper.
• tests/ contains offline fakes for the legacy Chat Completions shape.

Target shape

• In customer_support_bot/client.py, call client.responses.create(...) instead of client.chat.completions.create(...).
• Keep the same model argument.
• Replace the wrapper's messages argument with an input_items argument.
• In customer_support_bot/replies.py, pass the two-message system/user conversation as input_items.
• Forward input_items as the Responses API input argument.
• Keep temperature=0.
• Return response.output_text instead of completion.choices[0].message.content.
• Preserve the draft_reply(client, *, model, case_id, customer_message) function signature.
• Update client-wrapper and reply tests to fake the Responses API instead of Chat Completions.
• Tests must remain offline; do not import or instantiate the real OpenAI client.

Required validation pipeline

• Before editing, run baseline tests: python -m unittest discover -s tests -t ..
• After editing, run the compile/check command: python -m compileall -q customer_support_bot tests.
• After the compile/check command passes, run final tests: python -m unittest discover -s tests -t ..
• Validate with python -m unittest discover -s tests -t ..