Add idiomatic documentation to Go, Java, Python, and Rust clients

clients/java/mxgateway-cli/src/main/java/com/dohertylan/mxgateway/cli/MxGatewayCli.java

@@ -38,6 +38,10 @@ import picocli.CommandLine.Model.CommandSpec;
 import picocli.CommandLine.Option;
 import picocli.CommandLine.Spec;
 
+/**
+ * Picocli entry point for the {@code mxgw-java} test CLI used by the
+ * cross-language smoke matrix.
+ */
 @Command(
         name = "mxgw-java",
         mixinStandardHelpOptions = true,
@@ -48,6 +52,9 @@ public final class MxGatewayCli implements Callable<Integer> {
     @Spec
     private CommandSpec spec;
 
+    /**
+     * Builds a CLI bound to the default gRPC client factory.
+     */
     public MxGatewayCli() {
         this(new GrpcMxGatewayCliClientFactory());
     }
@@ -56,11 +63,25 @@ public final class MxGatewayCli implements Callable<Integer> {
         this.clientFactory = clientFactory;
     }
 
+    /**
+     * Process entry point.
+     *
+     * @param args command-line arguments
+     */
     public static void main(String[] args) {
         int exitCode = commandLine(new GrpcMxGatewayCliClientFactory()).execute(args);
         System.exit(exitCode);
     }
 
+    /**
+     * Test-friendly entry point that runs the CLI against the supplied
+     * {@link PrintWriter} pair instead of the system streams.
+     *
+     * @param out writer that receives standard output
+     * @param err writer that receives standard error
+     * @param args command-line arguments
+     * @return the picocli exit code
+     */
     public static int execute(PrintWriter out, PrintWriter err, String... args) {
         return execute(new GrpcMxGatewayCliClientFactory(), out, err, args);
     }
@@ -280,6 +301,9 @@ public final class MxGatewayCli implements Callable<Integer> {
         return values;
     }
 
+    /**
+     * Picocli subcommand that prints the client and protocol version numbers.
+     */
     @Command(name = "version", description = "Prints the Java client version.")
     public static final class VersionCommand implements Callable<Integer> {
         @Spec