Bind extension methods to the extended type so they are callable from the instance

[jhonabreul]

What does this implement/fix? Explain your changes.

C# extension methods were only reachable from Python through their static class, so the instance-style call every C# example uses failed:

AttributeError: 'OrderStatus' object has no attribute 'is_open'

Cause: ClassManager builds a type's Python class from its real members only, and extension methods are static methods on another type.

The change:

• ExtensionMethods indexes the public extension methods of each scanned assembly by the type they extend, inside AssemblyManager.ScanAssembly. Framework assemblies and Python.Runtime are skipped so LINQ is not grafted onto every collection.
• ClassManager.GetClassInfo binds the extensions of the type and of the interfaces it introduces as members, so snake-case aliases come for free. A real member with the same name wins, as in C#.
• MethodBinder.Bind treats an extension called through an instance like a bound operator: it matches the Python arguments against the remaining parameters and prepends the instance.
• Base-class extensions reach subclasses through the Python type hierarchy. Open generic targets (this T, this IEnumerable<T>) and by-ref targets are skipped because the binder cannot infer them from the instance.
• The index is cleared on AssemblyManager.Initialize so a re-initialized engine rebuilds it.

Limits: targets converted to Python natives (string, decimal, DateTime, double) cannot be bound, and their extensions stay available through the static class.

Does this close any currently open issues?

No.

Any other comments?

Tests, in TestExtensionMethods:

• Enum extension callable from the value in both snake and Pascal case, with positional and keyword arguments.
• Class extension callable from the instance, including a default parameter.
• Subclass inherits a base-class extension; interface extension is bound on the implementing class.
• Real member with the same name wins over the extension.
• Closed generic target (List<int>) is bound; open generic target is not.
• Static call forms through the extension class and through the extended class keep working.
• Python-native target (string) is not bound but its static form works.
• Full embedding suite: 1003 passed, 0 failed.