fix(acp): use SessionDB.replace_messages for atomic history rewrite

ACP's save_session() did a non-atomic clear_messages() + append_message()
loop. If any message hit an exception mid-loop (bad tool_call shape, etc.),
the DELETE had already committed and the persisted conversation was lost.

SessionDB.replace_messages() wraps DELETE + bulk INSERT in a single
BEGIN IMMEDIATE transaction that rolls back on any exception, so a bad
message can no longer clobber previously-persisted history.

Salvages @Awsh1's PR #13675 — uses the existing replace_messages()
helper (which covers more message fields than the PR's own copy)
instead of adding a duplicate.

acp_adapter/session.py

@@ -466,17 +466,10 @@ class SessionManager:
                 except Exception:
                     logger.debug("Failed to update ACP session metadata", exc_info=True)
 
-            # Replace stored messages with current history.
-            db.clear_messages(state.session_id)
-            for msg in state.history:
-                db.append_message(
-                    session_id=state.session_id,
-                    role=msg.get("role", "user"),
-                    content=msg.get("content"),
-                    tool_name=msg.get("tool_name") or msg.get("name"),
-                    tool_calls=msg.get("tool_calls"),
-                    tool_call_id=msg.get("tool_call_id"),
-                )
+            # Replace stored messages with current history atomically so a
+            # mid-rewrite failure rolls back and the previously persisted
+            # conversation is preserved (salvaged from #13675).
+            db.replace_messages(state.session_id, state.history)
         except Exception:
             logger.warning("Failed to persist ACP session %s", state.session_id, exc_info=True)