fix(tui): /model picker switch doesn't survive between jcode relaunches or new sessions

Problem

After switching a model in /model (via Ctrl+Tab/Ctrl+Shift+Tab or /model <name>), the selection does not survive between jcode relaunches or across new sessions. The model reverts to the config.toml [provider].default_model value on the next launch.

Root cause

The TUI model picker's finalize_model_switch() only persists the model to the session file (session.save()), not to config.toml. So:

• A resumed session restores the model from the session file (works).
• A new session or relaunch reads config.toml's [provider].default_model → reverts to the old default (bug).

This is especially visible for user-defined named OpenAI-compatible providers ([providers.<name>]) with model_catalog = false, where the /model list comes from config's [[providers.<name>.models]] entries loaded fresh on every launch.

Why the inline-interactive picker was already correct

The inline-interactive model picker (/model browse UI) already calls Config::set_default_model() to persist the selection to config.toml. Only the direct paths (cycle_model and /model <name>) missed this call.

Fix

Add persist_model_switch_to_config() which writes the active model + provider key to config.toml via Config::set_default_model, and call it from the two user-initiated switch paths (cycle_model, /model <name>). Failover- and auth-driven switches deliberately do NOT persist, so they cannot override the user's configured default.

Files changed (4 files, +192/-1)

Regression tests (both pass)

• cycle_model_persists_switch_to_config_toml — Writes config.toml with default_model = "model-a", creates a mock provider with available_models = ["model-a", "model-b"], cycles forward (a→b) and backward (b→a), reads config.toml back from disk and asserts default_model matches.
• model_command_persists_switch_to_config_toml — Same setup, uses /model model-b command path (handle_model_command), asserts config.toml's default_model is updated to model-b.

test tui::app::tests::cycle_model_persists_switch_to_config_toml ... ok
test tui::app::tests::model_command_persists_switch_to_config_toml ... ok

The tests verify the full end-to-end path: user-initiated switch → finalize_model_switch (session save) → persist_model_switch_to_config → Config::set_default_model → config.toml on disk → reload via Config::load() → assert persisted model matches.

Branch

Branch: https://github.com/alecuba16/jcode/tree/fix/model-persist-config
Commit: 9756b080

Single commit containing both the fix and the regression tests.

Start in crates/jcode-tui/src/tui/app/model_context.rs and inspect the model switch paths, then read crates/jcode-tui/src/tui/app/tests/model_persist_config.rs and its inclusion in tests.rs. Run the two named regression tests; done means user-initiated model changes persist in config.toml across reloads without persisting failover or auth-driven switches.