fix: Preserve set tags with known RHS values for multi-pass rendering

Enable deferred execution mode to ensure set tags are preserved even
when the RHS is fully resolved. This is needed for multi-pass rendering
where variables need to remain defined in subsequent render passes.

Example: {% set x = name %} with {name: "World"}
Before: World (x would be undefined in next pass)
After: {% set x = 'World' %}World (x remains defined)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: hs-lsong

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

@@ -37,5 +37,6 @@ public void prepareContext(Context context) {
     super.prepareContext(context);
     context.setExpressionStrategy(new PreserveUndefinedExpressionStrategy());
     context.setDynamicVariableResolver(varName -> DeferredValue.instance());
+    context.setDeferredExecutionMode(true);
   }
 }

src/test/java/com/hubspot/jinjava/mode/PreserveUndefinedExecutionModeTest.java

@@ -144,11 +144,11 @@ public void itHandlesNestedUndefinedInKnownStructure() {
   }
 
   @Test
-  public void itEvaluatesSetTagWithKnownRHSValue() {
+  public void itPreservesSetTagWithKnownRHSValue() {
     Map<String, Object> context = new HashMap<>();
     context.put("name", "World");
     String output = render("{% set x = name %}{{ x }}", context);
-    assertThat(output).isEqualTo("World");
+    assertThat(output).isEqualTo("{% set x = 'World' %}World");
   }
 
   @Test