a2ae9a1acf
Tightens the experimental OpenAI Codex (ChatGPT) provider so the v0.8.55 gate is green. - clippy: collapse 5 nested if/if-let blocks flagged by clippy::collapsible_if into let-chains (oauth.rs env-override resolution, responses.rs SSE delta handling). cargo clippy --workspace --all-targets -- -D warnings is now clean. - fmt: cargo fmt --all over the Codex/Together changes (the gate's --check was failing, incl. a mangled "| ApiProvider::Ollama"). - default model: Config::default_model() now resolves to the Codex default (gpt-5.5) for the Codex provider instead of leaking a DeepSeek default_text_model the Responses backend rejects. The carve-out sits after the explicit provider-scoped model block (so [providers.openai_codex] model still wins) and before the DeepSeek-validating path, which is unchanged. Adds a behavior test. https://claude.ai/code/session_013cHWv5sR6XPnVWzfMP8uma