Upgrade Servlet mock classes to Servlet 6.1

This commit upgrades our Mock Servlet classes for Servlet 6.1 support:

* the read/write `ByteBuffer` variants for `ServletInputStream` and
  `ServletOutputStream` were not added as the default implementation
  matches well the testing use case.
* Implement the session accessor with a simple lambda. Our mocks do not
  simulate the scheduling of request/response processing on different
  threads.
* Ensure that the response content length can only be written before the
  response is committed. Calling those methods after commit is a no-op,
  per specification.

Closes gh-33749

spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java

@@ -319,8 +319,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public void setContentLength(int contentLength) {
-		this.contentLength = contentLength;
-		doAddHeaderValue(HttpHeaders.CONTENT_LENGTH, contentLength, true);
+		if (!this.committed) {
+			this.contentLength = contentLength;
+			doAddHeaderValue(HttpHeaders.CONTENT_LENGTH, contentLength, true);
+		}
 	}
 
 	/**
@@ -334,8 +336,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public void setContentLengthLong(long contentLength) {
-		this.contentLength = contentLength;
-		doAddHeaderValue(HttpHeaders.CONTENT_LENGTH, contentLength, true);
+		if (!this.committed) {
+			this.contentLength = contentLength;
+			doAddHeaderValue(HttpHeaders.CONTENT_LENGTH, contentLength, true);
+		}
 	}
 
 	public long getContentLengthLong() {

spring-test/src/main/java/org/springframework/mock/web/MockHttpSession.java

@@ -45,7 +45,6 @@ import org.springframework.util.Assert;
  * @author Vedran Pavic
  * @since 1.0.2
  */
-@SuppressWarnings("deprecation")
 public class MockHttpSession implements HttpSession {
 
 	/**
@@ -240,6 +239,12 @@ public class MockHttpSession implements HttpSession {
 		return this.isNew;
 	}
 
+	@Override
+	public Accessor getAccessor() {
+		return sessionConsumer -> sessionConsumer.accept(MockHttpSession.this);
+	}
+
+
 	/**
 	 * Serialize the attributes of this session into an object that can be
 	 * turned into a byte array with standard Java serialization.

spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java

@@ -319,8 +319,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public void setContentLength(int contentLength) {
-		this.contentLength = contentLength;
-		doAddHeaderValue(HttpHeaders.CONTENT_LENGTH, contentLength, true);
+		if (!this.committed) {
+			this.contentLength = contentLength;
+			doAddHeaderValue(HttpHeaders.CONTENT_LENGTH, contentLength, true);
+		}
 	}
 
 	/**
@@ -334,8 +336,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public void setContentLengthLong(long contentLength) {
-		this.contentLength = contentLength;
-		doAddHeaderValue(HttpHeaders.CONTENT_LENGTH, contentLength, true);
+		if (!this.committed) {
+			this.contentLength = contentLength;
+			doAddHeaderValue(HttpHeaders.CONTENT_LENGTH, contentLength, true);
+		}
 	}
 
 	public long getContentLengthLong() {
@@ -636,7 +640,7 @@ public class MockHttpServletResponse implements HttpServletResponse {
 		sendRedirect(url, HttpServletResponse.SC_MOVED_TEMPORARILY, true);
 	}
 
-	// @Override - on Servlet 6.1
+	@Override
 	public void sendRedirect(String url, int sc, boolean clearBuffer) throws IOException {
 		Assert.state(!isCommitted(), "Cannot send redirect - response is already committed");
 		Assert.notNull(url, "Redirect URL must not be null");

spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpSession.java

@@ -45,7 +45,6 @@ import org.springframework.util.Assert;
  * @author Vedran Pavic
  * @since 1.0.2
  */
-@SuppressWarnings("deprecation")
 public class MockHttpSession implements HttpSession {
 
 	/**
@@ -239,6 +238,11 @@ public class MockHttpSession implements HttpSession {
 		return this.isNew;
 	}
 
+	@Override
+	public Accessor getAccessor() {
+		return sessionConsumer -> sessionConsumer.accept(MockHttpSession.this);
+	}
+
 	/**
 	 * Serialize the attributes of this session into an object that can be
 	 * turned into a byte array with standard Java serialization.