Fix Codex OAuth Responses proxy input normalization

[ruizanthony]

This PR fixes a Codex OAuth proxy compatibility issue when Agent Zero uses LiteLLM Responses API.

LiteLLM can send input as a string or empty string in some Responses API paths. The local Codex OAuth proxy forwarded that payload unchanged to the Codex upstream /responses endpoint, which expects input to be a list and returns {"detail":"Input must be a list"}.

The fix normalizes input inside prepare_responses_body() before forwarding:

• list: preserved;
• dict: wrapped in a one-item list;
• non-empty string: wrapped as a user message;
• empty string or None: converted to an empty list.

Regression tests cover string input and empty string input.

Checks run:

• /a0/venv/bin/python -m py_compile plugins/_oauth/helpers/codex.py plugins/_oauth/helpers/routes.py tests/test_oauth_codex.py
• /a0/venv/bin/python -m pytest tests/test_oauth_codex.py -q — 37 passed