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## Why Pre-sampling compaction intentionally uses the previous turn's model when the compaction compatibility hash changes or when switching to a model with a smaller context window. This keeps compaction aligned with the settings that produced the history, but it can block the next turn when a resumed ChatGPT thread still references a model slug that has since been retired. The Codex backend rejects that compaction request before the user's currently selected model gets a chance to sample. This PR lets those threads recover without changing previous-model compaction behavior for API-key authentication or custom providers. It is stacked on #31316, which is a behavior-preserving extraction of the individual remote compaction attempts; this PR contains the fallback behavior. ## What changed - For automatic previous-model compaction, capture the selected model's request context when using ChatGPT authentication with the OpenAI provider and the selected model differs from the previous model. - If the previous-model attempt returns an `InvalidRequest`, retry compaction once with the selected model for both `/responses/compact` and Responses Compaction V2. - Complete history processing, lifecycle events, and token accounting with the context of the model that successfully compacted the thread. - If the fallback also fails, return the original previous-model error so the retry does not change the user-visible failure. - Record fallback attempts with reason, implementation, and outcome telemetry. - Leave API-key authentication, custom providers, same-model turns, and non-`InvalidRequest` failures on their existing paths. ## Testing - `just test -p codex-core -E 'test(pre_sampling_compact) | test(model_unavailable_error)'` (10 tests) - Added integration coverage for a resumed thread whose model was renamed, a model downshift using Responses Compaction V2, and API-key authentication with a custom provider.