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.Tests/MxGatewayClientCliTests.cs

@@ -468,6 +468,141 @@ public sealed class MxGatewayClientCliTests
         Assert.Contains("\"objectCount\": 99", text);
     }
 
+    /// <summary>Verifies that batch mode executes a single no-gateway command and writes the EOR sentinel.</summary>
+    [Fact]
+    public async Task RunAsync_Batch_SingleVersionCommand_WritesOutputAndEorSentinel()
+    {
+        using var output = new StringWriter();
+        using var error = new StringWriter();
+        using var stdin = new StringReader("version --json\n");
+
+        int exitCode = await MxGatewayClientCli.RunAsync(
+            ["batch"],
+            output,
+            error,
+            clientFactory: null,
+            standardInput: stdin);
+
+        Assert.Equal(0, exitCode);
+        string text = output.ToString();
+        Assert.Contains("\"gatewayProtocolVersion\"", text);
+        Assert.Contains("__MXGW_BATCH_EOR__", text);
+        // Sentinel must appear after the output, not before.
+        int outputIdx = text.IndexOf("gatewayProtocolVersion", StringComparison.Ordinal);
+        int eorIdx = text.IndexOf("__MXGW_BATCH_EOR__", StringComparison.Ordinal);
+        Assert.True(outputIdx < eorIdx, "EOR sentinel must follow command output.");
+        Assert.Equal(string.Empty, error.ToString());
+    }
+
+    /// <summary>Verifies that batch mode processes two commands sequentially and writes two EOR sentinels.</summary>
+    [Fact]
+    public async Task RunAsync_Batch_TwoVersionCommands_WritesTwoEorSentinels()
+    {
+        using var output = new StringWriter();
+        using var error = new StringWriter();
+        // Two commands followed by EOF (end of string).
+        using var stdin = new StringReader("version\nversion --json\n");
+
+        int exitCode = await MxGatewayClientCli.RunAsync(
+            ["batch"],
+            output,
+            error,
+            clientFactory: null,
+            standardInput: stdin);
+
+        Assert.Equal(0, exitCode);
+        string text = output.ToString();
+        int firstEor = text.IndexOf("__MXGW_BATCH_EOR__", StringComparison.Ordinal);
+        int secondEor = text.IndexOf("__MXGW_BATCH_EOR__", firstEor + 1, StringComparison.Ordinal);
+        Assert.True(firstEor >= 0, "First EOR sentinel must be present.");
+        Assert.True(secondEor > firstEor, "Second EOR sentinel must follow first.");
+        Assert.Equal(string.Empty, error.ToString());
+    }
+
+    /// <summary>Verifies that batch mode on EOF (empty stdin) exits 0 immediately without writing any sentinel.</summary>
+    [Fact]
+    public async Task RunAsync_Batch_EmptyStdin_ExitsZeroWithNoOutput()
+    {
+        using var output = new StringWriter();
+        using var error = new StringWriter();
+        using var stdin = new StringReader(string.Empty);
+
+        int exitCode = await MxGatewayClientCli.RunAsync(
+            ["batch"],
+            output,
+            error,
+            clientFactory: null,
+            standardInput: stdin);
+
+        Assert.Equal(0, exitCode);
+        Assert.Equal(string.Empty, output.ToString());
+        Assert.Equal(string.Empty, error.ToString());
+    }
+
+    /// <summary>
+    ///     Verifies that batch mode continues after a command failure and writes the error JSON
+    ///     to stdout (not stderr), followed by the EOR sentinel.
+    /// </summary>
+    [Fact]
+    public async Task RunAsync_Batch_CommandFailure_WritesErrorJsonToStdoutAndContinues()
+    {
+        using var output = new StringWriter();
+        using var error = new StringWriter();
+        // First line: a gateway command with no API key (will fail).
+        // Second line: version (will succeed).
+        using var stdin = new StringReader("open-session --endpoint http://localhost:5000\nversion --json\n");
+
+        int exitCode = await MxGatewayClientCli.RunAsync(
+            ["batch"],
+            output,
+            error,
+            clientFactory: _ => throw new InvalidOperationException("injected failure"),
+            standardInput: stdin);
+
+        Assert.Equal(0, exitCode);
+        string text = output.ToString();
+
+        // Error record: the error JSON must be on stdout, not stderr.
+        Assert.Contains("\"error\"", text);
+        Assert.Equal(string.Empty, error.ToString());
+
+        // Both records must be present.
+        int firstEor = text.IndexOf("__MXGW_BATCH_EOR__", StringComparison.Ordinal);
+        int secondEor = text.IndexOf("__MXGW_BATCH_EOR__", firstEor + 1, StringComparison.Ordinal);
+        Assert.True(firstEor >= 0, "EOR after failed command must be present.");
+        Assert.True(secondEor > firstEor, "EOR after successful command must follow first EOR.");
+
+        // Second record must contain the version output.
+        string afterFirstEor = text[(firstEor + "__MXGW_BATCH_EOR__".Length)..];
+        Assert.Contains("\"gatewayProtocolVersion\"", afterFirstEor);
+    }
+
+    /// <summary>Verifies that batch mode treats an empty (blank) line as EOF and exits 0.</summary>
+    [Fact]
+    public async Task RunAsync_Batch_EmptyLine_ExitsZeroAfterPreviousCommands()
+    {
+        using var output = new StringWriter();
+        using var error = new StringWriter();
+        // One command, then an empty line (stop signal), then another command that must NOT run.
+        using var stdin = new StringReader("version --json\n\nversion --json\n");
+
+        int exitCode = await MxGatewayClientCli.RunAsync(
+            ["batch"],
+            output,
+            error,
+            clientFactory: null,
+            standardInput: stdin);
+
+        Assert.Equal(0, exitCode);
+        string text = output.ToString();
+        // Only one EOR sentinel — the second command after the empty line must not execute.
+        int firstEor = text.IndexOf("__MXGW_BATCH_EOR__", StringComparison.Ordinal);
+        int secondEor = text.IndexOf("__MXGW_BATCH_EOR__", firstEor + 1, StringComparison.Ordinal);
+        Assert.True(firstEor >= 0, "One EOR sentinel must be present.");
+        Assert.Equal(-1, secondEor);
+        Assert.Equal(string.Empty, error.ToString());
+    }
+
     /// <summary>Fake CLI client for testing.</summary>
     private sealed class FakeCliClient : IMxGatewayCliClient
     {