Fix OpenAI Codex provider errors when running agent.
Symptoms
- Codex requests failed with: "Instructions are not valid"
- Codex streaming failed on invalid UTF-8: "Cannot decode byte ..."
Fix
- Use Codex CLI instructions as
instructions payload and pass system prompt via developer message. - Decode SSE chunks with lenient UTF-8 decoding.
Acceptance
agent --provider=openai-codex --model=gpt-5.2-codex "ping" succeeds.
Fixed Codex instructions payload to use Codex CLI prompt text and switched SSE decoding to lenient UTF-8; agent ping now succeeds.