Add per-account OpenAI ChatGPT OAuth adapter

[CompleteDotTech]

Summary

• add a separate OpenAI (OAuth) LLM option to the adapter catalog
• implement server-side OpenAI device login and encrypted credential hand-off
• send ChatGPT Responses requests with per-adapter account credentials
• refresh and persist each account independently
• redact OAuth secrets and account identity from browser responses
• support multiple saved accounts by adapter instance name

Anthropic OAuth is intentionally out of scope for this PR and can follow in a separate change.

Validation

• uv run --project unstract/sdk1 pytest -q -p no:cacheprovider unstract/sdk1/tests/test_openai_oauth.py (10 passed)
• targeted Ruff checks passed
• targeted Biome check passed