fix(ml): replace unwrap() with ok_or/? in DQN IQN paths

Replace 6 unwrap() calls with safe error handling in DQN IQN code:
- Production: 3 unwrap() on iqn_network/iqn_target_network replaced with
  ok_or_else returning MLError::ModelError for clear diagnostics
- Tests: 2 result.unwrap() replaced with ?, 2 DQN::new().unwrap() replaced
  with ? after changing test signatures to return anyhow::Result<()>

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

backtesting/src/strategy_runner.rs

@@ -1361,22 +1361,16 @@ mod tests {
     }
 
     #[test]
-    fn test_production_features_have_51_dimensions() {
+    fn test_production_features_have_51_dimensions() -> Result<(), Box<dyn std::error::Error>> {
         let mut extractor = ProductionFeatureExtractorAdapter::new();
         // Feed 55 price updates (past the warmup period of 50)
         for i in 0..55 {
             let price = 100.0 + i as f64 * 0.1;
             let volume = 1000.0;
             let timestamp = Utc::now();
-            extractor
-                .update(price, volume, timestamp)
-                .unwrap_or_else(|e| {
-                    panic!("Production extractor update failed at step {}: {}", i, e)
-                });
+            extractor.update(price, volume, timestamp)?;
         }
-        let features = extractor
-            .extract_features()
-            .unwrap_or_else(|e| panic!("Production extractor extract_features failed: {}", e));
+        let features = extractor.extract_features()?;
         assert_eq!(
             features.len(),
             51,
@@ -1391,6 +1385,7 @@ mod tests {
                 i, val
             );
         }
+        Ok(())
     }
 
     #[test]