Ignore DOCTYPE inside a multi-line comment body

XmlValidationModeDetector peeks at the start of an XML document to
choose between DTD- and XSD-based validation, skipping any DOCTYPE that
appears inside an XML comment.

Prior to this commit, consumeCommentTokens() short-circuited a line
with no start or end comment marker by returning it unchanged, even
while already inside a multi-line comment. Such a body line was then
treated as content, so a literal "DOCTYPE" word in the comment body
caused an XSD document to be misdetected as DTD-based.

This commit honors the "in comment" parse state in that early return so
a comment body line is treated as empty content, completing the fix for
gh-27915 which only covered comment markers on the same line.

Closes gh-36948

Signed-off-by: junhyeong9812 <pickjog@gmail.com>

spring-core/src/main/java/org/springframework/util/xml/XmlValidationModeDetector.java

@@ -151,7 +151,9 @@ public class XmlValidationModeDetector {
 	private String consumeCommentTokens(String line) {
 		int indexOfStartComment = line.indexOf(START_COMMENT);
 		if (indexOfStartComment == -1 && !line.contains(END_COMMENT)) {
-			return line;
+			// If we are inside a multi-line comment, the entire line is comment
+			// data and must not be treated as content.
+			return (this.inComment ? "" : line);
 		}
 
 		String result = "";

spring-core/src/test/java/org/springframework/util/xml/XmlValidationModeDetectorTests.java

@@ -55,7 +55,8 @@ class XmlValidationModeDetectorTests {
 		"xsdWithNoComments.xml",
 		"xsdWithMultipleComments.xml",
 		"xsdWithDoctypeInComment.xml",
-		"xsdWithDoctypeInOpenCommentWithAdditionalCommentOnSameLine.xml"
+		"xsdWithDoctypeInOpenCommentWithAdditionalCommentOnSameLine.xml",
+		"xsdWithDoctypeInMultiLineCommentBody.xml"
 	})
 	void xsdDetection(String fileName) throws Exception {
 		assertValidationMode(fileName, VALIDATION_XSD);

spring-core/src/test/resources/org/springframework/util/xml/xsdWithDoctypeInMultiLineCommentBody.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+	See the DOCTYPE notes for legacy configs
+-->
+<beans xmlns="http://www.springframework.org/schema/beans"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="https://www.springframework.org/schema/beans
+		https://www.springframework.org/schema/beans/spring-beans.xsd">
+
+</beans>