Add regression tests for the critical Prisma-backed flows

[robertocarlous]

Why
Database behavior is central to the backend, especially for sessions, event persistence, and DLQ handling. Focused regression tests would catch schema or query regressions early.

Acceptance criteria

• Cover session creation and cleanup flows.
• Cover event persistence and cursor updates.
• Cover DLQ write/read paths and any retry logic.

[Menjay7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. Thank you!