Refactor PartGenerator to use isLast

This commit refactors the PartGenerator to use the newly introduced
Token::isLast property.

See gh-28006

spring-web/src/test/java/org/springframework/http/codec/multipart/DefaultPartHttpMessageReaderTests.java

@@ -118,6 +118,10 @@ class DefaultPartHttpMessageReaderTests  {
 		Flux<Part> result = reader.read(forClass(Part.class), request, emptyMap());
 
 		StepVerifier.create(result)
+				.consumeNextWith(part -> {
+					assertThat(part.headers().getFirst("Header")).isEqualTo("Value");
+					part.content().subscribe(DataBufferUtils::release);
+				})
 				.expectError(DecodingException.class)
 				.verify();
 	}