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