feat: Add user existence checks to economy commands and refactor trade service to expose sessions for testing.

src/commands/economy/balance.ts

@@ -23,6 +23,8 @@ export const balance = createCommand({
 
         const user = await userService.getOrCreateUser(targetUser.id, targetUser.username);
 
+        if (!user) throw new Error("Failed to retrieve user data.");
+
         const embed = createBaseEmbed(undefined, `**Balance**: ${user.balance || 0n} AU`, "Yellow")
             .setAuthor({ name: targetUser.username, iconURL: targetUser.displayAvatarURL() });