MEV analyzers: warn on polymorphic derived types registered via IJsonTypeInfoResolver

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

```csharp
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddValidation();
builder.Services.ConfigureHttpJsonOptions(options =>
{
var resolver = new DefaultJsonTypeInfoResolver();
resolver.Modifiers.Add(typeInfo =>
{
if (typeInfo.Type == typeof(MyBaseType))
{
typeInfo.PolymorphismOptions = new JsonPolymorphismOptions
{
TypeDiscriminatorPropertyName = "$type"
};
typeInfo.PolymorphismOptions.DerivedTypes.Add(
new JsonDerivedType(typeof(MyDerivedType1), "type1"));
typeInfo.PolymorphismOptions.DerivedTypes.Add(
new JsonDerivedType(typeof(MyDerivedType2), "type2"));
}
});
options.SerializerOptions.TypeInfoResolver = resolver;
});
```

If `MyDerivedType1` and `MyDerivedType2` don't have `[ValidatableType]` attribute and also didn't appear in the type graph that is statically known at compile-time, a warning should suggest adding the attribute to them. Otherwise, validation for them will be skipped if any endpoint encounters `MyBaseType`.

### Describe the solution you'd like

Analyzer warning to help with this case where validation is silently skipped.

### Additional context

_No response_

Locate the MEV analyzer implementation and its existing ValidatableType diagnostics and tests; begin by tracing how statically known types are discovered and how IJsonTypeInfoResolver registrations are represented. Done means the analyzer warns when resolver-registered derived types lack [ValidatableType] and are absent from the compile-time type graph, covering the shown polymorphic scenario.

Written by the indexing model from the issue text.