fix: OpenAPI schema incorrectly marks properties with default initializers as required

[Copilot]

Problem

RequiredPropertySchemaTransformer was marking all non-nullable reference types as required in OpenAPI schemas, including properties with default initializers. This caused OAuthAccount.ExtraData to appear as required when it shouldn't be:

public record OAuthAccount
{
    public required string Provider { get; init; }      // ✓ Should be required
    public required string ProviderUserId { get; init; } // ✓ Should be required  
    public required string Username { get; init; }       // ✓ Should be required
    
    public SettingsDictionary ExtraData { get; init; } = new(); // ✗ Was marked required, should be optional
}

Solution

Check for C# 11+ RequiredMemberAttribute instead of relying on nullability context:

// Before: Used nullability context (unreliable for properties with initializers)
var nullabilityInfo = nullabilityContext.Create(property);
return nullabilityInfo.WriteState == NullabilityState.NotNull;

// After: Check for explicit 'required' modifier
var requiredMemberAttribute = property.GetCustomAttribute<RequiredMemberAttribute>();
if (requiredMemberAttribute is not null)
    return true;

Reference types are now only marked as required if they have the explicit required modifier. Non-nullable value types remain required as before.

Original prompt

Reference: https://github.com/exceptionless/Exceptionless/actions/runs/21518290999/job/62002232212 find rca and apply fix

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