Make dev watch ignore more specific (#39504)
This fixes the case where we were incorrectly ignoring the next-auth API endpoint since it matched the `.next` watch ignore and also adds a test case to ensure we match this route correctly.
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
Fixes: https://github.com/vercel/next.js/issues/39495
