refactor(interpreter): support backend-specific code execution

[isaacbmiller]

Summary

Makes DSPy's existing code-execution modules work cleanly with constrained or remote CodeInterpreter implementations without adding a runtime abstraction or coupling core to any backend.

• route CodeAct tools through the existing mutable CodeInterpreter.tools capability boundary instead of copying host function source into the guest
• preserve closures, host policy, callable signatures, sync tools, and async tools during registration
• let interpreters optionally provide generated-code guidance through execution_instructions
• apply that guidance through immutable call-local RLM and CodeAct signatures, preserving persisted predictor schemas, demos, and concurrent calls
• describe the existing Deno/Pyodide environment through the same optional mechanism
• make non-UTF-8 SandboxSerializable transport independent of the base64 standard-library module by using bytes.fromhex
• forward allow_unsafe_lm_state to any custom Parameter.load_state implementation that accepts it, rather than special-casing Predict

Motivation

Implementing a second real interpreter backend exposed places where RLM and CodeAct accidentally depended on Deno/Pyodide details despite already accepting a generic CodeInterpreter factory. These changes keep backend ownership behind the existing interpreter interface and are independently useful for constrained, remote, and capability-based interpreters.

No Monty implementation or dependency is included in this PR.

Compatibility

• execution_instructions is optional and does not expand the required runtime-checkable CodeInterpreter protocol
• backend guidance is appended to a call-local signature; predictor field schemas and serialized state are unchanged
• interpreters without guidance retain existing behavior
• caller-owned and factory-owned interpreter lifecycle semantics are unchanged

Validation

• 120 passed, 58 skipped across RLM, CodeAct, and BaseModule focused tests
• Deno-enabled CodeAct: 15 passed
• Ruff passed on touched implementation and focused CodeAct tests
• git diff --check passed

Repository-wide collection is blocked in this orb by unrelated missing optional dependencies: numpy, datamodel_code_generator, and Pillow.

Review setup

The GitHub App token cannot create a PR directly against stanfordnlp/dspy. This fork draft targets monty-upstream-base, the exact upstream parent commit (0312f0d), so the diff contains only this upstreamable commit. The eventual target is stanfordnlp/dspy:main.