fix: refresh commit-message models after /model or /weak-model switch

[Shxiao101]

Fixes #5213

Problem

GitRepo only receives commit_message_models() when it is first constructed. /model and /weak-model raise SwitchCoder, and main.py rebuilds the coder via Coder.create(..., from_coder=old_coder) — but the new coder inherits the original repo object from from_coder.original_kwargs, so Coder.__init__ skips the GitRepo construction branch (self.repo is not None) and repo.models keeps pointing at the old [weak_model, main_model] pair. Commit messages — both manual /commit and the automatic post-edit ones — keep using the previous weak model until aider is restarted.

Change

While attaching an already-constructed repo in Coder.__init__, refresh repo.models from the current main_model:

if self.repo:
    self.root = self.repo.root
    # Refresh the commit-message models so /model and /weak-model
    # switches take effect for an already-constructed GitRepo.
    self.repo.models = main_model.commit_message_models()

repo.models is only consumed by GitRepo.get_commit_message(), and commit_message_models() is a pure accessor returning [self.weak_model, self], so the refresh is an idempotent no-op on first construction and cannot bypass any normalization (GitRepo.__init__ stores models as-is).

Testing

• Reproduction with a real GitRepo and two Coder.create() calls using different weak models: on main, repo.models stayed ['gpt-4o-mini', 'gpt-4o-mini'] after the switch; with this change it becomes ['gpt-3.5-turbo', 'gpt-4o-mini'].
• pytest tests/basic/test_repo.py tests/basic/test_coder.py: 58 passed, 5 skipped, 29 subtests passed — identical before and after the change.