## Description
Hardens the Captain model override preferences API so account-level
overrides follow the same feature-router contract used by runtime LLM
calls. The API now permits model and feature keys from `llm.yml`,
removes blank model overrides, rejects invalid saved model combinations,
and returns each feature's effective model, provider, and source for UI
clients.
Fixes https://linear.app/chatwoot/issue/CW-7425/test-new-models
## Type of change
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update
## How Has This Been Tested?
Verified the account preferences API and account model validation
behavior for valid overrides, invalid model values, unknown feature
keys, blank override removal, and effective model/provider/source
payload metadata.
- `eval "$(rbenv init -)" && bundle exec rspec
spec/controllers/api/v1/accounts/captain/preferences_controller_spec.rb
spec/models/account_spec.rb
spec/models/concerns/captain_featurable_spec.rb
spec/lib/llm/feature_router_spec.rb`
- `eval "$(rbenv init -)" && bundle exec rubocop
app/controllers/api/v1/accounts/captain/preferences_controller.rb
app/models/concerns/account_settings_schema.rb
app/models/concerns/captain_featurable.rb
spec/controllers/api/v1/accounts/captain/preferences_controller_spec.rb
spec/models/account_spec.rb
spec/models/concerns/captain_featurable_spec.rb
spec/lib/llm/feature_router_spec.rb`
- `git diff --check`
## Checklist:
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules
