docs: complete XML-doc coverage and strip internal tracking IDs from code comments

Resolve all CommentChecker findings across the gateway server, worker, tests,
and .NET client (314 -> 0 real issues): add missing <returns>/<summary>/<param>
on public and test members, convert Stream/interface overrides to <inheritdoc/>,
and remove internal task/issue tracking IDs (SEC-*, IPC-*, WRK-*, GWC-*, TST-*,
Client.Dotnet-*) from shipped code documentation while preserving the design
rationale prose. Shipped comments should not carry internal bookkeeping, and
complete XML docs keep the analyzer/TreatWarningsAsErrors gate and generated API
docs clean. The 6 remaining flags are heuristic false positives (MD5, UTC-4,
capacity-1, near-1601) left intact so real documentation is not corrupted.

Claude-Session: https://claude.ai/code/session_01DMXXvNuPekkkrTEyPNxEkW

src/ZB.MOM.WW.MxGateway.Tests/Metrics/GatewayMetricsTests.cs

@@ -161,7 +161,7 @@ public sealed class GatewayMetricsTests
     /// Verifies that <see cref="GatewayMetrics.HeartbeatFailed"/> increments
     /// <c>mxgateway.heartbeats.failed</c> without emitting a <c>session_id</c> tag: the tag is
     /// unbounded cardinality (every session mints a new exporter time series), so per-session
-    /// attribution is deliberately kept out of the exported counter (SEC-20).
+    /// attribution is deliberately kept out of the exported counter.
     /// </summary>
     [Fact]
     public void HeartbeatFailed_IncrementsCounterWithoutSessionIdTag()