Add message deletion from a session thread

[imDarshanGK]

Description: add message deletion from a session thread.

Keep the scope focused on the title and implement the smallest useful version first.
Include any tests, docs, or UI updates needed so the change is easy to verify.

[anandsundaramoorthysa]

Hi @imDarshanGK 👋

I'd like to work on this under SSoC 2026. Could you please assign it to me?

My plan to add message deletion from a session thread:

• Add a backend endpoint to delete a single message from a session (with ownership/validation checks), and remove any associated data so the thread stays consistent.
• Add a delete action in the chat UI (e.g. a delete option on a message) with a confirmation, and update the thread view after deletion.
• Keep scope to the smallest useful version first, and add a test to cover the delete flow.

Thanks!

[anandsundaramoorthysa]

Raised #395 for this. It adds single-message deletion: a session-scoped DELETE /api/sessions/{id}/messages/{message_id} endpoint (+ message_count sync), get_messages_full now returns message ids, and a per-message delete control in ChatWindow with an inline Delete? Yes/No confirm. Verified with the full pytest suite (73 passing, incl. 2 new tests) and a live end-to-end check (delete in the UI → removed from the thread and DB). Ready for review!