Fix: Remove duplicate enum values in HttpStatusCode dropdown for WriteHttpResponse

[zengande]

When generating the dropdown list for the status input in the WriteHttpResponse activity, duplicate enum values were included due to Enum.GetValues() returning multiple names with the same underlying numeric value from System.Net.HttpStatusCode.

This fix filters the list to include only distinct values, grouped by their underlying integer representation.

Related issue: #6845

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This change is