fix: address review findings in grid sampler, parallel optimization, and CSV export

- Rename stale "GridSearchSampler" panic message to "GridSampler"
- Assert concurrency > 0 in optimize_parallel to prevent deadlock
- Fix inaccurate comment in CSV export (empty for all non-complete trials, not just pruned)

src/sampler/grid.rs

@@ -601,7 +601,7 @@ impl Sampler for GridSampler {
         // Check if all points have been exhausted
         assert!(
             cached.current_index < cached.points.len(),
-            "GridSearchSampler: all grid points exhausted"
+            "GridSampler: all grid points exhausted"
         );
 
         // Get the current grid point and advance the index
@@ -908,7 +908,7 @@ mod tests {
     }
 
     #[test]
-    #[should_panic(expected = "GridSearchSampler: all grid points exhausted")]
+    #[should_panic(expected = "GridSampler: all grid points exhausted")]
     fn test_sampler_panics_after_exhaustion() {
         let sampler = GridSampler::new();
         let dist = Distribution::Categorical(CategoricalDistribution { n_choices: 2 });

src/study/async_impl.rs

@@ -171,6 +171,8 @@ where
         use tokio::sync::Semaphore;
         use tokio::task::JoinSet;
 
+        assert!(concurrency > 0, "concurrency must be at least 1");
+
         #[cfg(feature = "tracing")]
         let _span = tracing::info_span!("optimize_parallel", n_trials, concurrency, direction = ?self.direction).entered();
 

src/study/export.rs

@@ -91,7 +91,7 @@ where
         for trial in trials.iter() {
             write!(writer, "{}", trial.id)?;
 
-            // Value: empty for pruned trials.
+            // Value: empty for non-complete trials.
             if trial.state == TrialState::Complete {
                 write!(writer, ",{}", trial.value)?;
             } else {