fix(ai): fix memory leaks in ai session handling

Add missing `ai_session_unref` calls to prevent memory leaks.

- In `_ai_request_thread`, release session on error paths and
  after successful processing.
- In `cmd_ai_start`, release reference after passing ownership
  to the AI window.

src/command/cmd_funcs.c

@@ -10981,6 +10981,9 @@ cmd_ai_start(ProfWin* window, const char* const command, gchar** args)
     }
     log_debug("[AI-CMD] wins_new_ai() returned successfully, window type: %d", ai_win->type);
 
+    // Release the reference held by cmd_ai_start, since the window now holds one
+    ai_session_unref(session);
+
     // Add welcome message to the AI window
     log_debug("[AI-CMD] Adding welcome messages...");
     win_println(ai_win, THEME_DEFAULT, "-", "AI Chat: %s/%s", provider_name, model);