Create fresh extensions for Python instances

[zklapow]

Summary

• allow extension suppliers to create fresh handlers for each Python instance
• validate each instance import schema against the shared runtime image schema
• test concurrent instances with distinct handlers

Why

Extensions can carry instance-specific registries and handlers, but rebuilding a factory and runtime image for every instance is expensive. Suppliers keep one shared factory/image/executor while isolating extension state per instance.

Testing

• build_java(modules=["core", "tests"])