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/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli.Tests/WriteCommandRegionValidationTests.cs

@@ -21,6 +21,7 @@ public sealed class WriteCommandRegionValidationTests
     /// <param name="region">The read-only Modbus region to attempt a write against.</param>
     /// <param name="type">The data type used in the write attempt.</param>
     /// <param name="value">The raw string value supplied to the write command.</param>
+    /// <returns>A task that represents the asynchronous test operation.</returns>
     [Theory]
     [InlineData(ModbusRegion.DiscreteInputs, ModbusDataType.Bool, "0")]
     [InlineData(ModbusRegion.InputRegisters, ModbusDataType.UInt16, "1")]
@@ -44,6 +45,7 @@ public sealed class WriteCommandRegionValidationTests
 
     /// <summary>Verifies that Coils region requires Bool data type (Driver.Modbus.Cli-002).</summary>
     /// <param name="type">The non-Bool data type that should be rejected for the Coils region.</param>
+    /// <returns>A task that represents the asynchronous test operation.</returns>
     [Theory]
     [InlineData(ModbusDataType.UInt16)]
     [InlineData(ModbusDataType.Int16)]