Replace ErrorCodes with ReturnCode (#5624)

`ErrorCodes` in dotnet-trace and `ReturnCode` in Common are redundant.
`ReturnCode` is the established standard across diagnostic tools
(dotnet-trace, dotnet-counters) and provides better type safety as an
enum. `ErrorCodes` existed only for legacy ConvertCommand usage.

## Changes

- **ConvertCommand.cs**: Migrated from `ErrorCodes` to `ReturnCode`
  - Added `using Microsoft.Internal.Common.Utils`
- Replaced `ErrorCodes.ArgumentError` → `(int)ReturnCode.ArgumentError`
  - Replaced `ErrorCodes.UnknownError` → `(int)ReturnCode.UnknownError`

- **ErrorCode.cs**: Removed completely
  - No longer needed as ConvertCommand now uses ReturnCode directly

```diff
- return ErrorCodes.ArgumentError;
+ return (int)ReturnCode.ArgumentError;
```

This standardizes error code handling across all diagnostic tools.

## Testing

- ✅ Build succeeded with 0 warnings, 0 errors
- ✅ All 109 dotnet-trace unit tests passed

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<details>

<summary>Original prompt</summary>

> Can you assess the history of `ErrorCode` in
`src/Tools/dotnet-trace/CommandLine/Commands/ErrorCode.cs` and
`ReturnCode` in `src/Tools/Common/Commands/Utils.cs`. Can you assess
whether they are overlapping in responsibility, and if so, can you
deprecate ErrorCode in favor of ReturnCode?


</details>



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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: mdh1418 <[email protected]>

Author: Copilot

src/Tools/dotnet-trace/CommandLine/Commands/ConvertCommand.cs

@@ -8,6 +8,7 @@
 using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.Internal.Common;
+using Microsoft.Internal.Common.Utils;
 
 namespace Microsoft.Diagnostics.Tools.Trace
 {
@@ -21,12 +22,12 @@ public static int ConvertFile(TextWriter stdOut, TextWriter stdError, FileInfo i
             if (!Enum.IsDefined(format))
             {
                 stdError.WriteLine($"Please specify a valid option for the --format. Valid options are: {string.Join(", ", Enum.GetNames<TraceFileFormat>())}.");
-                return ErrorCodes.ArgumentError;
+                return (int)ReturnCode.ArgumentError;
             }
 
             if (!ValidateNetTraceHeader(stdError, inputFilename.FullName))
             {
-                return ErrorCodes.ArgumentError;
+                return (int)ReturnCode.ArgumentError;
             }
 
             string outputFilename = TraceFileFormatConverter.GetConvertedFilename(inputFilename.FullName, output?.FullName, format);
@@ -83,7 +84,7 @@ static int CopyNetTrace(TextWriter stdOut, TextWriter stdError, string inputfile
                 catch (Exception ex)
                 {
                     stdError.WriteLine($"Error copying nettrace to {outputfile}: {ex.Message}");
-                    return ErrorCodes.UnknownError;
+                    return (int)ReturnCode.UnknownError;
                 }
 
                 return 0;