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.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]