ref(core): Use conversation ID constant from conventions (#24966)

Uses `GEN_AI_CONVERSATION_ID` from `@sentry/conventions/attributes` in
the conversation-ID integration and its tests. Deprecates the existing
core constant while preserving its public export and the identical
`gen_ai.conversation.id` key.

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

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