Accept extensions when creating Python instances

[zklapow]

Summary

• add createInstance overloads that accept per-instance extensions
• keep builder extension suppliers for Wizer initialization and schema definition
• validate caller-provided extensions against the initialization import schema
• update concurrent instance tests to pass distinct handlers explicitly

Why

Callers need to bind request-specific registries and handlers without rebuilding the factory, runtime image, or executor. The shared image only requires a stable import schema; each instance can provide different implementations for that schema.

Testing

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