feat: Preserve block tags when extends path is undefined

When extends tag path is undefined in PreserveUndefinedExecutionMode,
block tags were being processed normally causing their content to be
extracted without the block wrapper. This fix adds an isExtendsDeferred
flag that gets set when ExtendsTag throws DeferredValueException, allowing
BlockTag to also defer and preserve its original syntax.

This enables proper multi-pass rendering where templates with undefined
extends paths preserve both the extends and block tags for later resolution.

🤖 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/interpret/Context.java

@@ -900,6 +900,14 @@ public void setPreserveComments(boolean preserveComments) {
     contextConfiguration = contextConfiguration.withPreserveComments(preserveComments);
   }
 
+  public boolean isExtendsDeferred() {
+    return contextConfiguration.isExtendsDeferred();
+  }
+
+  public void setExtendsDeferred(boolean extendsDeferred) {
+    contextConfiguration = contextConfiguration.withExtendsDeferred(extendsDeferred);
+  }
+
   public TemporaryValueClosable<Boolean> withUnwrapRawOverride() {
     TemporaryValueClosable<Boolean> temporaryValueClosable = new TemporaryValueClosable<>(
       isUnwrapRawOverride(),

src/main/java/com/hubspot/jinjava/interpret/ContextConfigurationIF.java

@@ -53,6 +53,11 @@ default boolean isPreserveComments() {
     return false;
   }
 
+  @Default
+  default boolean isExtendsDeferred() {
+    return false;
+  }
+
   @Default
   default ErrorHandlingStrategy getErrorHandlingStrategy() {
     return ErrorHandlingStrategy.of();

src/main/java/com/hubspot/jinjava/lib/tag/BlockTag.java

@@ -20,6 +20,7 @@
 import com.hubspot.jinjava.doc.annotations.JinjavaParam;
 import com.hubspot.jinjava.doc.annotations.JinjavaSnippet;
 import com.hubspot.jinjava.doc.annotations.JinjavaTextMateSnippet;
+import com.hubspot.jinjava.interpret.DeferredValueException;
 import com.hubspot.jinjava.interpret.JinjavaInterpreter;
 import com.hubspot.jinjava.interpret.TemplateSyntaxException;
 import com.hubspot.jinjava.tree.TagNode;
@@ -60,6 +61,14 @@ public class BlockTag implements Tag {
 
   @Override
   public OutputNode interpretOutput(TagNode tagNode, JinjavaInterpreter interpreter) {
+    if (interpreter.getContext().isExtendsDeferred()) {
+      throw new DeferredValueException(
+        "block tag",
+        tagNode.getLineNumber(),
+        tagNode.getStartPosition()
+      );
+    }
+
     HelperStringTokenizer tagData = new HelperStringTokenizer(tagNode.getHelpers());
     if (!tagData.hasNext()) {
       throw new TemplateSyntaxException(

src/main/java/com/hubspot/jinjava/lib/tag/ExtendsTag.java

@@ -89,6 +89,7 @@ public class ExtendsTag implements Tag {
   @Override
   public String interpret(TagNode tagNode, JinjavaInterpreter interpreter) {
     if (interpreter.getContext().isDeferredExecutionMode()) {
+      interpreter.getContext().setExtendsDeferred(true);
       throw new DeferredValueException("extends tag");
     }
     HelperStringTokenizer tokenizer = new HelperStringTokenizer(tagNode.getHelpers());
@@ -101,11 +102,18 @@ public String interpret(TagNode tagNode, JinjavaInterpreter interpreter) {
       );
     }
 
-    String path = interpreter.resolveString(
-      tokenizer.next(),
-      tagNode.getLineNumber(),
-      tagNode.getStartPosition()
-    );
+    String path;
+    try {
+      path =
+        interpreter.resolveString(
+          tokenizer.next(),
+          tagNode.getLineNumber(),
+          tagNode.getStartPosition()
+        );
+    } catch (DeferredValueException e) {
+      interpreter.getContext().setExtendsDeferred(true);
+      throw e;
+    }
     path = interpreter.resolveResourceLocation(path);
     interpreter
       .getContext()