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.ExternalSystemGateway/DatabaseGateway.cs

@@ -70,6 +70,7 @@ public class DatabaseGateway : IDatabaseGateway
     /// tests can substitute a connection whose <c>OpenAsync</c> fails.
     /// </summary>
     /// <param name="connectionString">The ADO.NET connection string.</param>
+    /// <returns>A new <see cref="DbConnection"/> for the given connection string.</returns>
     internal virtual DbConnection CreateConnection(string connectionString) =>
         new SqlConnection(connectionString);
 
@@ -145,6 +146,7 @@ public class DatabaseGateway : IDatabaseGateway
     /// </summary>
     /// <param name="message">The buffered store-and-forward message to deliver.</param>
     /// <param name="cancellationToken">Cancellation token for the delivery operation.</param>
+    /// <returns>A task that resolves to <c>true</c> on success, or <c>false</c> if the connection no longer exists.</returns>
     public async Task<bool> DeliverBufferedAsync(
         StoreAndForwardMessage message, CancellationToken cancellationToken = default)
     {
@@ -210,6 +212,12 @@ public class DatabaseGateway : IDatabaseGateway
     // precision for typical money/measurement values), and only fall through
     // to double for genuinely out-of-decimal-range values (very large
     // scientific-notation floats).
+    /// <summary>
+    /// Converts a <see cref="JsonElement"/> to the most appropriate SQL parameter value,
+    /// preferring <c>long</c> → <c>decimal</c> → <c>double</c> for numeric kinds to preserve precision.
+    /// </summary>
+    /// <param name="element">The JSON element to convert.</param>
+    /// <returns>A boxed CLR value suitable for use as an ADO.NET parameter, or <see cref="DBNull.Value"/> for null/undefined.</returns>
     internal static object JsonElementToParameterValue(JsonElement element) => element.ValueKind switch
     {
         JsonValueKind.String => (object?)element.GetString() ?? DBNull.Value,

src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ErrorClassifier.cs

@@ -17,6 +17,7 @@ public static class ErrorClassifier
     /// succeed and risks duplicate side effects.
     /// </summary>
     /// <param name="statusCode">The HTTP status code to classify.</param>
+    /// <returns><see langword="true"/> for 5xx, 408, or 429; <see langword="false"/> for all other status codes.</returns>
     public static bool IsTransient(HttpStatusCode statusCode)
     {
         var code = (int)statusCode;
@@ -27,6 +28,7 @@ public static class ErrorClassifier
     /// Determines whether an exception represents a transient failure.
     /// </summary>
     /// <param name="exception">The exception to classify.</param>
+    /// <returns><see langword="true"/> for connection/timeout/cancellation exceptions; <see langword="false"/> otherwise.</returns>
     public static bool IsTransient(Exception exception)
     {
         return exception is HttpRequestException
@@ -40,6 +42,7 @@ public static class ErrorClassifier
     /// </summary>
     /// <param name="message">Human-readable failure description.</param>
     /// <param name="inner">Optional inner exception that caused the transient failure.</param>
+    /// <returns>A new <see cref="TransientExternalSystemException"/> wrapping the given details.</returns>
     public static TransientExternalSystemException AsTransient(string message, Exception? inner = null)
     {
         return new TransientExternalSystemException(message, inner);

src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ServiceCollectionExtensions.cs

@@ -17,6 +17,7 @@ public static class ServiceCollectionExtensions
     /// Registers the External System Gateway services, HTTP client factory, and options.
     /// </summary>
     /// <param name="services">The service collection to configure.</param>
+    /// <returns>The same <see cref="IServiceCollection"/> for chaining.</returns>
     public static IServiceCollection AddExternalSystemGateway(this IServiceCollection services)
     {
         services.AddOptions<ExternalSystemGatewayOptions>()
@@ -49,6 +50,7 @@ public static class ServiceCollectionExtensions
     /// Placeholder for External System Gateway Akka.NET actor registrations (handled in AkkaHostedService).
     /// </summary>
     /// <param name="services">The service collection to configure.</param>
+    /// <returns>The same <see cref="IServiceCollection"/> for chaining.</returns>
     public static IServiceCollection AddExternalSystemGatewayActors(this IServiceCollection services)
     {
         // WP-10: Actor registration happens in AkkaHostedService.