Model steering incorrectly reverts to Gemini 2.5 Flash after Plan Mode when using 'auto' alias #21868

When using the auto model alias (or specialized auto aliases like auto-gemini-3), the CLI incorrectly steers to the Gemini 2.5 Flash model during the implementation phase (after exiting Plan Mode), even when Gemini 3 variants are active and preferred.

The transition from the planning phase (which correctly utilizes high-reasoning Gemini 3 models) to the implementation phase should maintain model family consistency. Instead, the current logic often causes an unexpected downgrade to the older Gemini 2.5 Flash model. This happens because the steering strategy for Plan Mode relies on hardcoded model constants and incomplete checks that do not account for the generic auto alias, Gemini 3.1 launch status, or specialized "custom tools" variants.

Steps to Reproduce:

1. Start a session with gemini and select Auto (Gemini 3) .
2. Enter Plan Mode and finalize a plan (observed resolving to Gemini 3.1 Pro).
3. Approve the plan to enter the implementation phase.
4. Observe that the model switches to Gemini 2.5 Flash instead of the expected Gemini 3 Flash.

Expected Behavior:
The CLI should maintain model family consistency (e.g., staying within Gemini 3.x) when transitioning from planning to implementation, ensuring that speed optimizations don't come at the cost of using an outdated model generation.