feat: Harden model override preferences (5/6) (#14846)

## 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

spec/controllers/api/v1/accounts/captain/preferences_controller_spec.rb

@@ -45,6 +45,28 @@ RSpec.describe 'Api::V1::Accounts::Captain::Preferences', type: :request do
         expect(json_response).to have_key(:models)
         expect(json_response).to have_key(:features)
       end
+
+      it 'returns effective model provider and source for each feature' do
+        account.update!(captain_models: { 'editor' => 'gpt-4.1' })
+
+        get "/api/v1/accounts/#{account.id}/captain/preferences",
+            headers: admin.create_new_auth_token,
+            as: :json
+
+        expect(response).to have_http_status(:success)
+        expect(json_response.dig(:features, :editor)).to include(
+          model: 'gpt-4.1',
+          selected: 'gpt-4.1',
+          provider: 'openai',
+          source: 'account_override'
+        )
+        expect(json_response.dig(:features, :label_suggestion)).to include(
+          model: Llm::Models.default_model_for('label_suggestion'),
+          selected: Llm::Models.default_model_for('label_suggestion'),
+          provider: 'openai',
+          source: 'default'
+        )
+      end
     end
   end
 
@@ -84,6 +106,43 @@ RSpec.describe 'Api::V1::Accounts::Captain::Preferences', type: :request do
         expect(account.reload.captain_models['editor']).to eq('gpt-4.1-mini')
       end
 
+      it 'does not persist unknown captain model feature keys' do
+        put "/api/v1/accounts/#{account.id}/captain/preferences",
+            headers: admin.create_new_auth_token,
+            params: { captain_models: { editor: 'gpt-4.1-mini', unknown_feature: 'gpt-4.1' } },
+            as: :json
+
+        expect(response).to have_http_status(:success)
+        expect(account.reload.captain_models).to eq('editor' => 'gpt-4.1-mini')
+      end
+
+      it 'rejects invalid captain model values for the feature' do
+        put "/api/v1/accounts/#{account.id}/captain/preferences",
+            headers: admin.create_new_auth_token,
+            params: { captain_models: { label_suggestion: 'gpt-5.1' } },
+            as: :json
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(json_response[:message]).to include('not a valid model for label_suggestion')
+        expect(account.reload.captain_models).to be_nil
+      end
+
+      it 'removes blank captain model overrides' do
+        account.update!(captain_models: { 'editor' => 'gpt-4.1' })
+
+        put "/api/v1/accounts/#{account.id}/captain/preferences",
+            headers: admin.create_new_auth_token,
+            params: { captain_models: { editor: '' } },
+            as: :json
+
+        expect(response).to have_http_status(:success)
+        expect(account.reload.captain_models).to be_nil
+        expect(json_response.dig(:features, :editor)).to include(
+          selected: Llm::Models.default_model_for('editor'),
+          source: 'default'
+        )
+      end
+
       it 'updates captain_models for document FAQ generation' do
         put "/api/v1/accounts/#{account.id}/captain/preferences",
             headers: admin.create_new_auth_token,