fix(providers): list gpt-5.5 for ChatGPT OAuth

[nguyenha935]

Summary

• add GPT-5.5 to the ChatGPT OAuth model catalog
• add reasoning metadata so the model picker exposes reasoning controls
• extend provider model API coverage to assert GPT-5.5 is listed

Testing

• docker run --rm -v "/root/.config/superpowers/worktrees/Goclaw/chatgpt-oauth-gpt-5-5:/src" -w /src golang:1.26-bookworm go test ./internal/http -run TestProvidersHandlerListProviderModelsChatGPTOAuthIncludesReasoningMetadata