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/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,