ref(core): Use status message constant from conventions (#24967)

Uses `SENTRY_STATUS_MESSAGE` from `@sentry/conventions/attributes` in
span serialization and existing constant-based tests. Deprecates the
existing core constant while preserving its public export and the
identical `sentry.status.message` key.

Part of #24961 / [JS-3868](https://linear.app/getsentry/issue/JS-3868).

Co-authored-by: GPT-6 <codex@openai.com>