Ensure StringDecoder supports multiline delimiters

This commit makes sure the StringDecoder supports stripping off
multi-line delimiters, such as \r\n. Specifically, we ensure that the
delimiter is stripped from the joined buffer.

Closes gh-26511

spring-core/src/test/java/org/springframework/core/codec/StringDecoderTests.java

@@ -125,10 +125,10 @@ class StringDecoderTests extends AbstractDecoderTests<StringDecoder> {
 		);
 
 		testDecode(input, String.class, step -> step
-				.expectNext("")
+				.expectNext("").as("1st")
 				.expectNext("abc")
 				.expectNext("defghi")
-				.expectNext("")
+				.expectNext("").as("2nd")
 				.expectNext("jklmno")
 				.expectNext("pqr")
 				.expectNext("stuvwxyz")
@@ -136,6 +136,21 @@ class StringDecoderTests extends AbstractDecoderTests<StringDecoder> {
 				.verify());
 	}
 
+	@Test
+	void decodeNewlinesAcrossBuffers()  {
+		Flux<DataBuffer> input = Flux.just(
+				stringBuffer("\r"),
+				stringBuffer("\n"),
+				stringBuffer("xyz")
+		);
+
+		testDecode(input, String.class, step -> step
+				.expectNext("")
+				.expectNext("xyz")
+				.expectComplete()
+				.verify());
+	}
+
 	@Test
 	void maxInMemoryLimit() {
 		Flux<DataBuffer> input = Flux.just(