Nullable collection coercion not applied to constructor parameters

[superyyrrzz]

Bug

ForgeMap v1.6 correctly applies nullable-safe collection coercion (via .ToDictionary() adapter) for property assignments, but does NOT apply it for constructor parameter expressions. This causes CS8620 under TreatWarningsAsErrors.

Reproduction

// Source (no nullable annotations):
public class Source
{
    public IDictionary<string, object> Metadata { get; set; }
}

// Destination (constructor-based, with nullable annotations):
public class Dest
{
    public Dest(IReadOnlyDictionary<string, object?>? metadata = null)
    {
        Metadata = metadata ?? new Dictionary<string, object?>();
    }
    public IReadOnlyDictionary<string, object?> Metadata { get; }
}

[ForgeMap(NullPropertyHandling = NullPropertyHandling.CoalesceToNew)]
public partial class TestForger
{
    public partial Dest Forge(Source source);
}

Generated code (constructor path):

return new Dest(
    metadata: new ReadOnlyDictionary<string, object>(source.Metadata)  // CS8620!
);

Expected: Should apply the same .ToDictionary() nullable adapter as property assignment:

return new Dest(
    metadata: Enumerable.ToDictionary(
        Enumerable.Select(source.Metadata, kv => new KeyValuePair<string, object?>(kv.Key, (object?)kv.Value)),
        kv => kv.Key, kv => kv.Value)
);

Impact

This prevents using constructor mapping for types with IReadOnlyDictionary<string, object?> constructor parameters when the source uses IDictionary<string, object>. The user must keep a manual mapping method.

Environment

• ForgeMap 1.6.0
• .NET 8.0 with TreatWarningsAsErrors