Fix Codex OAuth Responses streaming

[MTG-Thomas]

Summary

• route Codex backend generate() calls through the existing streaming Responses transport
• fold streamed Responses events back into the normal GenerateResponse shape
• add a mock SSE regression test covering Codex generate() with text/event-stream headers

Fixes #739.

Verification

cargo test -p stakai providers::openai::provider::tests

I also validated a patched CLI against a ChatGPT/Codex OAuth account with:

stakpak -p --max-steps 1 --model gpt-5.2 "Say OK only."

which returned OK instead of Stream must be set to true.