Adds `gpt-5.3-codex` as a built-in model under the OpenAI provider for
BYOK usage.
Model specs:
- 400,000 context window
- 128,000 max output tokens
- Reasoning token support (default medium effort)
- Uses the Responses API (like other codex models)
- Token counting falls back to the gpt-5 tokenizer
Closes AI-59
Release Notes:
- Added support for GPT-5.3-Codex as a bring-your-own-key model in the
OpenAI provider.