e2e: drive each client CLI through one long-lived batch process

The cross-language e2e matrix spawned one CLI process per operation —
~250 per client — paying a process (and, for the Java CLI, a full JVM)
cold-start every time. The Java leg alone ran ~16 minutes.

Each client CLI (dotnet, go, rust, python, java) gains a `batch`
subcommand: a single process that reads one command line from stdin,
runs it through the normal subcommand dispatch, writes the JSON result,
then a line containing exactly `__MXGW_BATCH_EOR__`. A failing command
writes its `{"error":...}` envelope and the loop continues.

run-client-e2e-tests.ps1 now launches one batch process per client and
pings every operation through its stdin/stdout, so startup is paid once
per client. The orchestration and assertions are unchanged; the parity
and auth phases now read the `{"error":...}` envelope instead of a
process exit code.

Full 5-client matrix with -VerifyWrite: ~15 min, down from ~35; the Java
leg dropped from ~16 min to ~2-3.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

clients/dotnet/MxGateway.Client.Cli/MxGatewayClientCli.cs

@@ -25,7 +25,7 @@ public static class MxGatewayClientCli
         TextWriter standardOutput,
         TextWriter standardError)
     {
-        return RunAsync(args, standardOutput, standardError)
+        return RunAsync(args, standardOutput, standardError, clientFactory: null, standardInput: null)
             .GetAwaiter()
             .GetResult();
     }
@@ -35,11 +35,13 @@ public static class MxGatewayClientCli
     /// <param name="standardOutput">TextWriter for command output.</param>
     /// <param name="standardError">TextWriter for error messages.</param>
     /// <param name="clientFactory">Optional factory to create the gateway client; defaults to MxGatewayClient.Create.</param>
+    /// <param name="standardInput">Optional TextReader for batch-mode stdin; defaults to <see cref="Console.In"/>.</param>
     public static Task<int> RunAsync(
         string[] args,
         TextWriter standardOutput,
         TextWriter standardError,
-        Func<MxGatewayClientOptions, IMxGatewayCliClient>? clientFactory = null)
+        Func<MxGatewayClientOptions, IMxGatewayCliClient>? clientFactory = null,
+        TextReader? standardInput = null)
     {
         ArgumentNullException.ThrowIfNull(args);
         ArgumentNullException.ThrowIfNull(standardOutput);
@@ -49,14 +51,19 @@ public static class MxGatewayClientCli
             args,
             standardOutput,
             standardError,
-            clientFactory ?? CreateDefaultClient);
+            clientFactory ?? CreateDefaultClient,
+            standardInput ?? Console.In);
     }
 
+    private const string BatchEndOfRecord = "__MXGW_BATCH_EOR__";
+
     private static async Task<int> RunCoreAsync(
         string[] args,
         TextWriter standardOutput,
         TextWriter standardError,
-        Func<MxGatewayClientOptions, IMxGatewayCliClient> clientFactory)
+        Func<MxGatewayClientOptions, IMxGatewayCliClient> clientFactory,
+        TextReader standardInput,
+        bool forceJsonErrors = false)
     {
         if (args.Length is 0 || IsHelp(args[0]))
         {
@@ -65,6 +72,12 @@ public static class MxGatewayClientCli
         }
 
         string command = args[0].ToLowerInvariant();
+
+        if (command is "batch")
+        {
+            return await RunBatchAsync(standardOutput, clientFactory, standardInput).ConfigureAwait(false);
+        }
+
         CliArguments arguments = new(args.Skip(1));
 
         try
@@ -142,7 +155,7 @@ public static class MxGatewayClientCli
             string? apiKey = TryResolveApiKey(arguments);
             string message = MxGatewayCliSecretRedactor.Redact(exception.Message, apiKey);
 
-            if (arguments.HasFlag("json"))
+            if (forceJsonErrors || arguments.HasFlag("json"))
             {
                 standardError.WriteLine(JsonSerializer.Serialize(
                     new { error = message, type = exception.GetType().Name },
@@ -239,6 +252,82 @@ public static class MxGatewayClientCli
         return cancellation;
     }
 
+    /// <summary>
+    ///     Runs the CLI in batch mode: reads one command line at a time from
+    ///     <paramref name="standardInput"/>, dispatches it through the normal
+    ///     routing, writes all output to <paramref name="standardOutput"/>, and
+    ///     then appends <see cref="BatchEndOfRecord"/> as a sentinel so the
+    ///     caller can delimit command results. Continues on failure; errors are
+    ///     written as JSON to <paramref name="standardOutput"/> (not stderr) so
+    ///     that the harness sees them inside the same delimited block. Exits 0
+    ///     on EOF or empty line.
+    /// </summary>
+    private static async Task<int> RunBatchAsync(
+        TextWriter standardOutput,
+        Func<MxGatewayClientOptions, IMxGatewayCliClient> clientFactory,
+        TextReader standardInput)
+    {
+        while (true)
+        {
+            string? line = await standardInput.ReadLineAsync().ConfigureAwait(false);
+
+            // EOF or empty line signals clean exit.
+            if (line is null || line.Length is 0)
+            {
+                return 0;
+            }
+
+            // Split on runs of ASCII whitespace — no quoting support by design.
+            string[] lineArgs = line.Split((char[]?)null, StringSplitOptions.RemoveEmptyEntries);
+
+            // Per-command output is buffered so we can redirect errors to stdout.
+            using StringWriter commandOutput = new();
+
+            // Errors in batch mode go to stdout (same delimited block), formatted as JSON.
+            // We use a capturing error writer and re-emit through commandOutput after the
+            // command returns, so the EOR sentinel always follows the complete result.
+            using StringWriter commandError = new();
+
+            try
+            {
+                await RunCoreAsync(
+                        lineArgs,
+                        commandOutput,
+                        commandError,
+                        clientFactory,
+                        standardInput,
+                        forceJsonErrors: true)
+                    .ConfigureAwait(false);
+            }
+            catch (Exception exception) when (exception is not OperationCanceledException)
+            {
+                // Unexpected exception that escaped RunCoreAsync (shouldn't happen, but be safe).
+                commandError.WriteLine(JsonSerializer.Serialize(
+                    new { error = exception.Message, type = exception.GetType().Name },
+                    JsonOptions));
+            }
+
+            // Write any buffered normal output first.
+            string commandOutputText = commandOutput.ToString();
+            if (commandOutputText.Length > 0)
+            {
+                standardOutput.Write(commandOutputText);
+            }
+
+            // Then any error output — in batch mode it belongs on stdout so the harness
+            // sees it inside the delimited record.
+            string commandErrorText = commandError.ToString();
+            if (commandErrorText.Length > 0)
+            {
+                standardOutput.Write(commandErrorText);
+            }
+
+            // Write the end-of-record sentinel and flush so the harness can unblock.
+            standardOutput.WriteLine(BatchEndOfRecord);
+            await standardOutput.FlushAsync().ConfigureAwait(false);
+        }
+    }
+
     private static Task<int> OpenSessionAsync(
         CliArguments arguments,
         IMxGatewayCliClient client,
@@ -1861,6 +1950,7 @@ public static class MxGatewayClientCli
 
     private static void WriteUsage(TextWriter writer)
     {
+        writer.WriteLine("mxgw-dotnet batch  (reads commands from stdin; writes output + __MXGW_BATCH_EOR__ after each)");
         writer.WriteLine("mxgw-dotnet version [--json]");
         writer.WriteLine("mxgw-dotnet ping --session-id <id> [--json]");
         writer.WriteLine("mxgw-dotnet open-session [--client-name <name>] [--json]");