fix(tui): restore auto model state on session load

[LING71671]

Summary

Closes #1797.

Resuming or explicitly loading a session saved while /model auto was active could restore the literal model string auto without restoring the matching auto_model state. The next turn could then treat auto as a concrete provider model and send it to the API.

Changes

• Route /load through App::set_model_selection so persisted model metadata rebuilds the full model-selection state.
• Clear stale effective auto-model/auto-reasoning labels when the model selection is restored.
• Restore thinking mode to Auto whenever the selected model is auto.
• Add regression coverage for explicit /load and apply_loaded_session resume paths.

Verification

• cargo fmt --all -- --check
• git diff --check
• cargo test -p deepseek-tui --bin deepseek-tui load_auto_model_session_restores_auto_mode
• cargo test -p deepseek-tui --bin deepseek-tui apply_loaded_session_restores_auto_model_mode
• cargo build

Note: cargo build still reports the existing unused migration marker warnings in schema_migration.rs; this change does not introduce new warnings.

[gemini-code-assist]

Code Review

This pull request updates the session loading mechanism to use the set_model_selection method, ensuring that application state—specifically reasoning_effort and last_effective_reasoning_effort—is correctly managed when restoring a session. It also introduces new test cases and updates existing ones to verify that the "auto" model selection mode is properly restored and that the internal state remains consistent. I have no feedback to provide.

[github-actions]

Thanks @LING71671 — your contribution landed in 02d4605868a1 on main:

fix(tui): restore auto model state on session load

Closing this PR now that the code is on main. Credit lives in the commit message and (where applicable) the CHANGELOG.md entry for the next release. Apologies for not closing this at the time of the merge — the auto-close workflow is new in v0.8.31.

If you want to land more work and would prefer your future PRs merge cleanly without a harvest step, the CONTRIBUTING.md doc has a short note on what makes a contribution mergeable as-is.