Flush writer after writing generated code (#5919)

BufferedWriter is created in a try-with-resources, so it is always
closed, but closing the writer does not actually flush the output
stream; it just flushes the remaining data *buffered* in BufferedWriter
to the stream:

https://github.com/openjdk/jdk/blob/3c72c04de7a43d265dae7160fe53baaaa8ae6f73/src/java.base/share/classes/java/io/BufferedWriter.java#L311-L322

We still need to call BufferedWriter#flush() explicitly so that the
output stream is flushed:

https://github.com/openjdk/jdk/blob/3c72c04de7a43d265dae7160fe53baaaa8ae6f73/src/java.base/share/classes/java/io/BufferedWriter.java#L298-L308

This should hopefully fix issues where
CodeWriter#validateFileContentMatches fails when comparing contents
because the file on disk is truncated.

Author: dagnir

codegen/src/main/java/software/amazon/awssdk/codegen/emitters/CodeWriter.java

@@ -119,6 +119,7 @@ public void flush() {
             if (fileSize(outputFile) == 0) {
                 try (BufferedWriter writer = Files.newBufferedWriter(outputFile, StandardCharsets.UTF_8)) {
                     writer.write(formattedContents);
+                    writer.flush();
                 }
             } else {
                 validateFileContentMatches(outputFile, formattedContents);