Eliminate all CS8618, CS0618, and ASP0019 compiler warnings

[Copilot]

This PR eliminates all compiler and analyzer warnings mentioned in issue #7044 by implementing proper nullability annotations, replacing obsolete API calls, and using recommended header dictionary methods.

Changes Made

1. Fixed Non-nullable Property Warnings (CS8618)

File: eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs

Added the required modifier to three non-nullable string properties to ensure they are initialized at object creation:

public required string Name { get; set; }
public required string GroupUid { get; set; }
public required string Description { get; set; }

This leverages C# 11's required members feature, which is the recommended approach for ensuring non-nullable reference types are properly initialized while maintaining model binding compatibility.

2. Replaced Obsolete HTML Parser Method (CS0618)

File: eFormAPI.Web/Services/WordProcessor.cs

Updated the AddHtml method to use the newer ParseBody API instead of the deprecated ParseHtml:

// Before: converter.ParseHtml(html);
converter.ParseBody(html);

The ParseBody method provides better output clarification and is the recommended replacement for the obsolete ParseHtml method.

3. Fixed IHeaderDictionary Usage (ASP0019)

Files:

• eFormAPI.Web/Controllers/Eforms/AudioController.cs
• eFormAPI.Web/Hosting/Security/ClaimsTransformer.cs

Replaced Headers.Add() calls with indexer syntax to prevent potential ArgumentException when adding duplicate keys:

// Before: Response.Headers.Add("Accept-Ranges", "bytes");
Response.Headers["Accept-Ranges"] = "bytes";

// Before: _httpContextAccessor.HttpContext.Response.Headers.Add(AuthConsts.UpdateHeaderName, AuthConsts.UpdateHeaderValue);
_httpContextAccessor.HttpContext.Response.Headers[AuthConsts.UpdateHeaderName] = AuthConsts.UpdateHeaderValue;

The indexer approach is safer as it sets or overwrites headers instead of throwing exceptions on duplicates.

Results

• ✅ Build succeeds with 0 code warnings
• ✅ All changes are minimal and focused on the specific warnings
• ✅ No breaking changes to existing functionality
• ✅ Code now follows current .NET 9.0 best practices

Fixes #7044

Original prompt

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This section details on the original issue you should resolve

<issue_title>Eleminate all warnings</issue_title>
<issue_description>We need to eliminate all warnings and implement correct code and still pass tests:

test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L36
Non-nullable property 'Description' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L35
Non-nullable property 'GroupUid' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L34
Non-nullable property 'Name' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Controllers/Eforms/AudioController.cs#L75
Use IHeaderDictionary.Append or the indexer to append or set headers. IDictionary.Add will throw an ArgumentException when attempting to add a duplicate key. (https://aka.ms/aspnet/analyzers)
test-dotnet: eFormAPI/eFormAPI.Web/Hosting/Security/ClaimsTransformer.cs#L89
Use IHeaderDictionary.Append or the indexer to append or set headers. IDictionary.Add will throw an ArgumentException when attempting to add a duplicate key. (https://aka.ms/aspnet/analyzers)
test-dotnet: eFormAPI/eFormAPI.Web/Hosting/Security/ClaimsTransformer.cs#L80
Use IHeaderDictionary.Append or the indexer to append or set headers. IDictionary.Add will throw an ArgumentException when attempting to add a duplicate key. (https://aka.ms/aspnet/analyzers)
test-dotnet: eFormAPI/eFormAPI.Web/Services/WordProcessor.cs#L62
'HtmlConverter.ParseHtml(string, CancellationToken)' is obsolete: 'Use ParseBody instead for output clarification'
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L36
Non-nullable property 'Description' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L35
Non-nullable property 'GroupUid' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L34
Non-nullable property 'Name' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #7047

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