fix(tests): add missing TradingState import to DQN trainer tests

The test module used `super::*` but TradingState lives in
crate::dqn, not in the trainer module — broke `cargo test --lib`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/ml/src/trainers/dqn/trainer/tests.rs

@@ -1,4 +1,5 @@
 use super::*;
+use crate::dqn::TradingState;
 use crate::hyperopt::ParameterSpace;
 use std::sync::OnceLock;