GH-9129: SFTP: Remove extra / for the file_remoteDirectory header

Fixes: #9129

If `.remoteDirectory("/sftpSource")`, then `FileHeaders.REMOTE_DIRECTORY` is `//sftpSource`

* Fix `AbstractInboundFileSynchronizer` to not add `/` if one is already present in the beginning of the `remoteDirectoryPath`

(cherry picked from commit d87a7098ca)

spring-integration-file/src/main/java/org/springframework/integration/file/remote/synchronizer/AbstractInboundFileSynchronizer.java

@@ -505,7 +505,10 @@ public abstract class AbstractInboundFileSynchronizer<F>
 				try {
 					String remoteFileMetadata =
 							new URI(protocol(), null, host, Integer.parseInt(port),
-									'/' + remoteDirectoryPath, null, remoteFileName)
+									remoteDirectoryPath.charAt(0) == '/'
+											? remoteDirectoryPath :
+											'/' + remoteDirectoryPath,
+									null, remoteFileName)
 									.toString();
 					this.remoteFileMetadataStore.put(buildMetadataKey(localFile), remoteFileMetadata);
 				}

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/dsl/SftpTests.java

@@ -75,7 +75,7 @@ public class SftpTests extends SftpTestSupport {
 		IntegrationFlow flow = IntegrationFlow
 				.from(Sftp.inboundAdapter(sessionFactory())
 								.preserveTimestamp(true)
-								.remoteDirectory("sftpSource")
+								.remoteDirectory("/sftpSource")
 								.regexFilter(".*\\.txt$")
 								.localFilenameExpression("#this.toUpperCase() + '.a'")
 								.localDirectory(getTargetLocalDirectory())
@@ -91,6 +91,7 @@ public class SftpTests extends SftpTestSupport {
 		File file = (File) payload;
 		assertThat(file.getName()).isEqualTo(" SFTPSOURCE1.TXT.a");
 		assertThat(file.getAbsolutePath()).contains("localTarget");
+		assertThat(message.getHeaders()).containsEntry(FileHeaders.REMOTE_DIRECTORY, "/sftpSource");
 
 		message = out.receive(10_000);
 		assertThat(message).isNotNull();