marshal: allow runtime bags to preserve raw code bytes - #8552

Summary

• add a MarshalBag::code_units_from_bytes hook for runtime-specific decoding of marshalled co_code
• add a make_code_with_constants_and_bytes hook so a runtime can retain the exact public byte string
• preserve the existing strict CodeUnits behavior by default

Motivation

Runtime code objects may accept raw opcode bytes that are not representable by the compiler-core Instruction enum. CPython and PyPy allow such code objects to round-trip through marshal; execution rejects the unknown opcode later. The current shared deserializer rejects these bytes before a runtime bag can preserve them.

These defaulted hooks keep compiler-core behavior unchanged while allowing runtimes with a raw-byte fallback to implement that behavior without duplicating the marshal parser.

Validation

• cargo fmt --all -- --check
• cargo test -p rustpython-compiler-core

This is a draft while the downstream runtime integration is validated.

Summary by CodeRabbit

• New Features
  • Added customizable handling for converting serialized code bytes into executable code units.
  • Added a construction hook that can access both decoded constants and the original code bytes during deserialization.
  • Preserved existing default behavior for standard runtime code loading.