fix(feedback): enforce a max message size from all sources

[aliu39]

Closes https://github.com/getsentry/sentry/issues/76298 Closes SENTRY-3B86

Decided on a limit of 4096, generously below the LLM request, postgres, and kafka size limits described in the ticket. Messages that are too large will be truncated (or rejected, for crash report modal) and skip spam detection, auto-marking as spam.

Includes a small refactor of create_feedback.py, moving stuff around and commenting a bit. ~~Renamed auto_ignore_spam_feedback to set_feedback_ignored.~~