- added a shared helper `take_active_draft_initial_content` to resolve
initial content
- `draft_prompt` is copied over to a new thread and is persisted in old
thread with this behavior


Self-Review Checklist:
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] no Unsafe blocks 
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable

Closes #38028 

Release Notes:

- persist draft messages in ai thread when a new thread is created so we
dont use any messages or draft prompts mistakenly

Question: 
need some guidance on how to persist a thread when the `draft_prompt` is
present but the message is unsent because currently having a draft
prompt in the agent input box and thenmaking a new agent thread copies
the prompt over to the new thread(changes from this pr) but on going
back to the old thread new thread is destroyed