docs: complete XML doc comments via fixdocs (2757 to 131 findings)

Add missing <returns>/<param>/<summary>/<typeparam> tags and clean up
misused inheritdoc across 481 files so the documented API surface is
complete. Documentation-only (zero code lines changed). The 131 remaining
findings are inheritdoc-style warnings deliberately left to preserve
hand-written implementation rationale (plan-decision notes, race-condition
explanations).

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/OpcUaApplicationHostTests.cs

@@ -19,6 +19,7 @@ public sealed class OpcUaApplicationHostTests : IDisposable
         $"otopcua-pki-{Guid.NewGuid():N}");
 
     /// <summary>Verifies StartAsync creates a self-signed certificate in the PKI own store.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
     public async Task StartAsync_creates_application_certificate_in_pki_own()
     {
@@ -41,6 +42,7 @@ public sealed class OpcUaApplicationHostTests : IDisposable
     }
 
     /// <summary>Verifies StartAsync reuses an existing certificate on the second boot.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
     public async Task StartAsync_reuses_existing_certificate_on_second_boot()
     {