ref(core): Use idle span finish reason constant from conventions (#24970)

Uses `SENTRY_IDLE_SPAN_FINISH_REASON` from
`@sentry/conventions/attributes` in idle-span handling and existing
constant-based tests. Deprecates the existing core constant while
preserving its public export and the identical
`sentry.idle_span_finish_reason` key.

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

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