refactor(tests): split integration.rs into focused subfolders

- Delete 20 duplicate tests already covered by parameter_tests.rs
- Move 11 pure Trial unit tests into src/trial.rs
- Split remaining 84 integration tests into tests/study/ (9 modules)
- Group sampler tests into tests/sampler/ (7 modules)
- Group pruner tests into tests/pruner/ (2 modules)

tests/study/summary.rs

@@ -0,0 +1,71 @@
+use optimizer::parameter::{FloatParam, Parameter};
+use optimizer::sampler::random::RandomSampler;
+use optimizer::{Direction, Error, Study};
+
+#[test]
+fn test_summary_with_completed_trials() {
+    let study: Study<f64> = Study::with_sampler(Direction::Minimize, RandomSampler::with_seed(1));
+    let x = FloatParam::new(0.0, 10.0).name("x");
+
+    study
+        .optimize(5, |trial| {
+            let val = x.suggest(trial)?;
+            Ok::<_, Error>(val * val)
+        })
+        .unwrap();
+
+    let summary = study.summary();
+    assert!(summary.contains("Minimize"));
+    assert!(summary.contains("5 trials"));
+    assert!(summary.contains("Best value:"));
+    assert!(summary.contains("x = "));
+}
+
+#[test]
+fn test_summary_no_completed_trials() {
+    let study: Study<f64> = Study::new(Direction::Maximize);
+    let summary = study.summary();
+    assert!(summary.contains("Maximize"));
+    assert!(summary.contains("0 trials"));
+    assert!(!summary.contains("Best value:"));
+}
+
+#[test]
+fn test_summary_with_pruned_trials() {
+    let study: Study<f64> = Study::with_sampler(Direction::Minimize, RandomSampler::with_seed(1));
+    let x = FloatParam::new(0.0, 10.0).name("x");
+
+    // Manually create some complete and pruned trials
+    for _ in 0..3 {
+        let mut trial = study.create_trial();
+        let val = x.suggest(&mut trial).unwrap();
+        study.complete_trial(trial, val);
+    }
+    for _ in 0..2 {
+        let mut trial = study.create_trial();
+        let _ = x.suggest(&mut trial).unwrap();
+        study.prune_trial(trial);
+    }
+
+    let summary = study.summary();
+    // Should show breakdown when there are pruned trials
+    if study.n_pruned_trials() > 0 {
+        assert!(summary.contains("complete"));
+        assert!(summary.contains("pruned"));
+    }
+}
+
+#[test]
+fn test_display_matches_summary() {
+    let study: Study<f64> = Study::with_sampler(Direction::Minimize, RandomSampler::with_seed(1));
+    let x = FloatParam::new(0.0, 10.0).name("x");
+
+    study
+        .optimize(3, |trial| {
+            let val = x.suggest(trial)?;
+            Ok::<_, Error>(val)
+        })
+        .unwrap();
+
+    assert_eq!(format!("{study}"), study.summary());
+}