docs: complete XML doc coverage (returns, summaries, inheritdoc)

Resolve all 622 issues flagged by the enhanced CommentChecker: add missing
<returns> tags (incl. the standard phrasing on non-generic Task methods),
add missing <summary> tags, and replace misused/redundant <inheritdoc/> on
members that override or implement nothing with real documentation.
Documentation-only — no behavior change; solution builds clean.

src/ZB.MOM.WW.ScadaBridge.Commons/Serialization/MxGatewayEndpointConfigSerializer.cs

@@ -21,11 +21,13 @@ public static class MxGatewayEndpointConfigSerializer
 
     /// <summary>Serializes a config to the typed JSON shape.</summary>
     /// <param name="config">The endpoint configuration to serialize.</param>
+    /// <returns>A camelCase JSON string representing the endpoint configuration.</returns>
     public static string Serialize(MxGatewayEndpointConfig config)
         => JsonSerializer.Serialize(config, JsonOpts);
 
     /// <summary>Parses stored config JSON; null/blank/malformed yields a default config.</summary>
     /// <param name="json">The stored JSON string.</param>
+    /// <returns>The deserialized <see cref="MxGatewayEndpointConfig"/>, or a default instance if the input is null, blank, or malformed.</returns>
     public static MxGatewayEndpointConfig Deserialize(string? json)
     {
         if (string.IsNullOrWhiteSpace(json)) return new MxGatewayEndpointConfig();
@@ -35,6 +37,7 @@ public static class MxGatewayEndpointConfigSerializer
 
     /// <summary>Flattens the typed config to the key-value shape the adapter consumes.</summary>
     /// <param name="c">The endpoint configuration to flatten.</param>
+    /// <returns>A string-keyed dictionary containing all endpoint configuration properties.</returns>
     public static IDictionary<string, string> ToFlatDict(MxGatewayEndpointConfig c) => new Dictionary<string, string>
     {
         ["Endpoint"] = c.Endpoint,
@@ -49,6 +52,7 @@ public static class MxGatewayEndpointConfigSerializer
 
     /// <summary>Reconstructs a config from the flat key-value shape; invalid numerics fall back to defaults.</summary>
     /// <param name="d">The flat dictionary.</param>
+    /// <returns>A <see cref="MxGatewayEndpointConfig"/> populated from the dictionary; missing or invalid entries use default values.</returns>
     public static MxGatewayEndpointConfig FromFlatDict(IDictionary<string, string> d)
     {
         var c = new MxGatewayEndpointConfig();