docs(src): add missing XML docs and strip tracking-ID comments

Sweep of 203 source files resolving CommentChecker findings: add
<summary>/<param>/<returns>/<inheritdoc> where missing, and remove
resolved task/issue tracking markers (Tests-NNN, Worker-NNN, Server-NNN,
Task N) from code comments. Comment/doc-only — no logic changes.
Server+Tests build clean under TreatWarningsAsErrors.

src/ZB.MOM.WW.MxGateway.Server/Security/Authentication/ApiKeyConstraintSerializer.cs

@@ -12,6 +12,7 @@ public static class ApiKeyConstraintSerializer
 
     /// <summary>Serializes API key constraints to JSON, or returns null if the constraints are empty.</summary>
     /// <param name="constraints">The constraints to serialize.</param>
+    /// <returns>The serialized JSON, or <see langword="null"/> when the constraints are empty.</returns>
     public static string? Serialize(ApiKeyConstraints constraints)
     {
         ArgumentNullException.ThrowIfNull(constraints);
@@ -20,6 +21,7 @@ public static class ApiKeyConstraintSerializer
 
     /// <summary>Deserializes API key constraints from JSON, or returns empty constraints if JSON is null or whitespace.</summary>
     /// <param name="json">The JSON string to deserialize.</param>
+    /// <returns>The deserialized constraints, or <see cref="ApiKeyConstraints.Empty"/> when <paramref name="json"/> is null/whitespace.</returns>
     public static ApiKeyConstraints Deserialize(string? json)
     {
         if (string.IsNullOrWhiteSpace(json))