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.NotificationService/SmtpTlsMode.cs

@@ -30,6 +30,7 @@ public static class SmtpTlsModeParser
     /// </summary>
     /// <param name="tlsMode">The TLS mode string to parse (None, StartTLS, or SSL); null/empty defaults to StartTLS.</param>
     /// <exception cref="ArgumentException">The value is not one of None/StartTLS/SSL.</exception>
+    /// <returns>The corresponding <see cref="SmtpTlsMode"/> enum value.</returns>
     public static SmtpTlsMode Parse(string? tlsMode)
     {
         if (string.IsNullOrWhiteSpace(tlsMode))