Improve error handling with user-friendly messages and classification (#1591)

* Improve chat LLM error messages

* Fix chat visibility regression in tests

* Harden chat error handling for review feedback

* Fix rubocop private method indentation

* Fix nil presentable_error_message, i18n strings, bare rescue

- Guard `presentable_error_message` with `return nil if error.blank?` so
  chats with no error return nil instead of the fallback string; this
  prevents the API serialisers from emitting a spurious error message and
  stops the mobile polling guard from firing on every successful chat
- Move all hardcoded user-facing error strings into
  config/locales/models/chat/en.yml and reference them via I18n.t()
- Replace bare `rescue` in `error_message_for` with `rescue StandardError`
  to avoid swallowing system-level exceptions
- Update tests to reference I18n keys instead of raw strings, and add
  tests for the nil-error case and the unrecognized-error fallback

https://claude.ai/code/session_01YFMjEds5WVyKPL42xBqMCX

---------

Co-authored-by: SureBot <sure-bot@we-promise.com>
Co-authored-by: Claude <noreply@anthropic.com>

app/views/api/v1/chats/_chat.json.jbuilder

@@ -2,6 +2,6 @@
 
 json.id chat.id
 json.title chat.title
-json.error chat.error.present? ? chat.error : nil
+json.error chat.presentable_error_message
 json.created_at chat.created_at.iso8601
 json.updated_at chat.updated_at.iso8601

app/views/api/v1/chats/index.json.jbuilder

@@ -5,7 +5,7 @@ json.chats @chats do |chat|
   json.title chat.title
   json.last_message_at chat.messages.ordered.first&.created_at&.iso8601
   json.message_count chat.messages.count
-  json.error chat.error.present? ? chat.error : nil
+  json.error chat.presentable_error_message
   json.created_at chat.created_at.iso8601
   json.updated_at chat.updated_at.iso8601
 end