Refactor eTag formatting into utility method

See gh-33412

spring-web/src/main/java/org/springframework/http/ETag.java

@@ -134,6 +134,15 @@ public record ETag(String tag, boolean weak) {
 		return result;
 	}
 
+	public static String format(String etag) {
+		if (!etag.startsWith("\"") && !etag.startsWith("W/\"")) {
+			etag = "\"" + etag;
+		}
+		if (!etag.endsWith("\"")) {
+			etag = etag + "\"";
+		}
+		return etag;
+	}
 
 	private enum State {
 

spring-web/src/main/java/org/springframework/http/ResponseEntity.java

@@ -570,12 +570,7 @@ public class ResponseEntity<T> extends HttpEntity<T> {
 		@Override
 		public BodyBuilder eTag(@Nullable String etag) {
 			if (etag != null) {
-				if (!etag.startsWith("\"") && !etag.startsWith("W/\"")) {
-					etag = "\"" + etag;
-				}
-				if (!etag.endsWith("\"")) {
-					etag = etag + "\"";
-				}
+				etag = ETag.format(etag);
 			}
 			this.headers.setETag(etag);
 			return this;