fix: Suppress method hiding warning on instance() method

The static instance() method intentionally hides the parent class method
to return the correct singleton for this execution mode, matching the
pattern used in NonRevertingEagerExecutionMode.

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: hs-lsong

src/main/java/com/hubspot/jinjava/mode/PreserveUndefinedExecutionMode.java

@@ -3,6 +3,7 @@
 import com.hubspot.jinjava.interpret.Context;
 import com.hubspot.jinjava.interpret.DeferredValue;
 import com.hubspot.jinjava.lib.expression.PreserveUndefinedExpressionStrategy;
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
 
 /**
  * An execution mode that preserves unknown/undefined variables as their original template syntax
@@ -31,6 +32,10 @@ public class PreserveUndefinedExecutionMode extends EagerExecutionMode {
 
   protected PreserveUndefinedExecutionMode() {}
 
+  @SuppressFBWarnings(
+    value = "HSM_HIDING_METHOD",
+    justification = "Purposefully overriding to return static instance of this class."
+  )
   public static ExecutionMode instance() {
     return INSTANCE;
   }