fix(sessions): keep named-pipe socket paths inside the macOS sun_path limit (NEXT-01)

The pipe name mxaccess-gateway-{pid}-session-{32hex} plus .NET's
CoreFxPipe_ prefix overflowed the 104-byte Unix-domain-socket path limit
under the default per-user macOS TMPDIR (~49 chars), so every test that
opened a real pipe threw ArgumentOutOfRangeException at pipe creation
unless TMPDIR=/tmp was exported. Rename to mxgw-{pid}-{sessionUid} (the
session guid hex without the session- prefix; worst-case 43 chars) and
shorten the three test-fixture names the same way. Uniqueness is
unchanged: gateway pid + full session guid. The worker receives the pipe
name via its launch command line, so mixed Server/Worker deploy SHAs are
unaffected. Docs updated in the same change (gateway.md,
GatewayProcessDesign, GatewayConfiguration, Sessions, CLAUDE.md); new
regression test pins the format and the length budget.

Verified: SessionManagerTests 39/39; SessionWorkerClientFactory,
GatewayEndToEndFakeWorkerSmoke, WorkerClient, and ReconnectReplay suites
33/33 under the default macOS TMPDIR — this also retires the
previously-misdiagnosed 'macOS pipe-timeout test failures': they were
this path-length throw, not a timeout-message defect.

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionManager.cs

@@ -417,7 +417,8 @@ public sealed class SessionManager : ISessionManager
         string? clientIdentity,
         string? ownerKeyId)
     {
-        string sessionId = CreateSessionId();
+        string sessionUid = Guid.NewGuid().ToString("N");
+        string sessionId = $"session-{sessionUid}";
         string backendName = string.IsNullOrWhiteSpace(request.RequestedBackend)
             ? GatewayContractInfo.DefaultBackendName
             : request.RequestedBackend!;
@@ -425,7 +426,11 @@ public sealed class SessionManager : ISessionManager
         TimeSpan startupTimeout = TimeSpan.FromSeconds(_options.Worker.StartupTimeoutSeconds);
         TimeSpan shutdownTimeout = TimeSpan.FromSeconds(_options.Worker.ShutdownTimeoutSeconds);
         TimeSpan leaseDuration = TimeSpan.FromSeconds(_options.Sessions.DefaultLeaseSeconds);
-        string pipeName = $"mxaccess-gateway-{Environment.ProcessId}-{sessionId}";
+        // The short prefix and bare guid keep the pipe's Unix-domain-socket path
+        // (TMPDIR + "CoreFxPipe_" + name) inside the 104-byte sun_path limit on
+        // macOS, whose default per-user TMPDIR is ~49 chars; the gateway PID keeps
+        // the name collision-free across gateway restarts (NEXT-01).
+        string pipeName = $"mxgw-{Environment.ProcessId}-{sessionUid}";
         string nonce = CreateNonce();
         DateTimeOffset openedAt = _timeProvider.GetUtcNow();
         string clientCorrelationId = CreateClientCorrelationId(request.ClientSessionName, sessionId);
@@ -484,11 +489,6 @@ public sealed class SessionManager : ISessionManager
             : timeout;
     }
 
-    private static string CreateSessionId()
-    {
-        return $"session-{Guid.NewGuid():N}";
-    }
-
     private static string CreateNonce()
     {
         Span<byte> bytes = stackalloc byte[32];

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/SessionManagerTests.cs

@@ -37,6 +37,36 @@ public sealed class SessionManagerTests
         Assert.Equal(1, metrics.GetSnapshot().SessionsOpened);
     }
 
+    /// <summary>
+    ///     Verifies the pipe name stays short enough that its Unix-domain-socket path
+    ///     (TMPDIR + "CoreFxPipe_" + name) fits the 104-byte macOS sun_path limit under the
+    ///     default per-user TMPDIR (~49 chars), and keeps the pid + session-guid uniqueness
+    ///     contract (NEXT-01).
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task OpenSessionAsync_PipeNameIsShortAndUniquePerPidAndSession()
+    {
+        FakeWorkerClient workerClient = new();
+        FakeSessionWorkerClientFactory factory = new(workerClient)
+        {
+            ApplyLifecycleTransitions = true,
+        };
+        SessionManager manager = CreateManager(factory);
+
+        GatewaySession session = await manager.OpenSessionAsync(CreateOpenRequest(), "client-1", ownerKeyId: null, CancellationToken.None);
+
+        Assert.Matches($"^mxgw-{Environment.ProcessId}-[0-9a-f]{{32}}$", session.PipeName);
+        Assert.EndsWith(session.SessionId["session-".Length..], session.PipeName, StringComparison.Ordinal);
+
+        // 104-byte sun_path − NUL − ~49-char default macOS TMPDIR − "CoreFxPipe_".
+        const int MaxPipeNameLength = 104 - 1 - 49 - 11;
+        int worstCasePidDigits = 5 - Environment.ProcessId.ToString(System.Globalization.CultureInfo.InvariantCulture).Length;
+        Assert.True(
+            session.PipeName.Length + Math.Max(0, worstCasePidDigits) <= MaxPipeNameLength,
+            $"Pipe name '{session.PipeName}' would overflow the macOS socket-path budget at a 5-digit pid.");
+    }
+
     /// <summary>Verifies that a session opened by an authenticated caller records that caller's API key id in OwnerKeyId.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/SessionWorkerClientFactoryFakeWorkerTests.cs

@@ -138,7 +138,7 @@ public sealed class SessionWorkerClientFactoryFakeWorkerTests : IAsyncDisposable
         return new GatewaySession(
             FakeWorkerHarness.DefaultSessionId,
             GatewayContractInfo.DefaultBackendName,
-            $"mxaccessgw-session-fake-worker-{Guid.NewGuid():N}",
+            $"mxgw-sf-{Guid.NewGuid():N}",
             FakeWorkerHarness.DefaultNonce,
             "test-client",
             "fake-worker-session-test",

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Workers/Fakes/FakeWorkerHarness.cs

@@ -60,7 +60,7 @@ public sealed class FakeWorkerHarness : IAsyncDisposable
         int maxMessageBytes = WorkerFrameProtocolOptions.DefaultMaxMessageBytes,
         CancellationToken cancellationToken = default)
     {
-        string pipeName = $"mxaccessgw-fake-worker-{Guid.NewGuid():N}";
+        string pipeName = $"mxgw-fw-{Guid.NewGuid():N}";
         NamedPipeServerStream gatewayStream = new(
             pipeName,
             PipeDirection.InOut,

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Workers/WorkerClientTests.cs

@@ -1069,7 +1069,7 @@ public sealed class WorkerClientTests
         /// <returns>The connected <see cref="PipePair"/>.</returns>
         public static async Task<PipePair> CreateAsync()
         {
-            string pipeName = $"mxaccessgw-workerclient-tests-{Guid.NewGuid():N}";
+            string pipeName = $"mxgw-wc-{Guid.NewGuid():N}";
             NamedPipeServerStream gatewayStream = new(
                 pipeName,
                 PipeDirection.InOut,