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];