fix: Sanitize query canceled API errors (#14933)

# Pull Request Template

## Description

Message creation API failures caused by PostgreSQL query cancellations
now return a generic retryable error instead of exposing raw database
internals such as `PG::QueryCanceled`, tuple identifiers, or relation
names to customers. Existing validation failures continue to return
their specific validation messages.

Closes
[CW-7538](https://linear.app/chatwoot/issue/CW-7538/do-not-expose-pgquerycanceled-details-in-messages-api-errors)

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] 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?

Reproduced the messages API path by raising
`ActiveRecord::QueryCanceled` from `Messages::MessageBuilder` and
verified the response contains the customer-safe localized error without
`PG::QueryCanceled` details.

Validation run locally:

- `bundle exec rspec
spec/controllers/api/v1/accounts/conversations/messages_controller_spec.rb`
- `bundle exec rubocop
app/controllers/concerns/request_exception_handler.rb
spec/controllers/api/v1/accounts/conversations/messages_controller_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
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] 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/conversations/messages_controller_spec.rb

@@ -51,6 +51,22 @@ RSpec.describe 'Conversation Messages API', type: :request do
         expect(json_response['error']).to eq('Validation failed: Content is too long (maximum is 150000 characters)')
       end
 
+      it 'returns a customer-safe error when the database query is canceled' do
+        message_builder = instance_double(Messages::MessageBuilder)
+        allow(Messages::MessageBuilder).to receive(:new).and_return(message_builder)
+        allow(message_builder).to receive(:perform)
+          .and_raise(ActiveRecord::QueryCanceled, 'PG::QueryCanceled: ERROR: canceling statement due to statement timeout')
+
+        post api_v1_account_conversation_messages_url(account_id: account.id, conversation_id: conversation.display_id),
+             params: { content: 'test-message', private: true },
+             headers: agent.create_new_auth_token,
+             as: :json
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(response.parsed_body['error']).to eq(I18n.t('errors.database.query_canceled'))
+        expect(response.parsed_body['error']).not_to include('PG::QueryCanceled')
+      end
+
       it 'creates an outgoing text message with a specific bot sender' do
         agent_bot = create(:agent_bot)
         time_stamp = Time.now.utc.to_s