[codex] Move flmexec Python default local

[k82cn]

Summary

Move the flmexec Python fallback version into flmexec itself and stop exporting that runtime policy from the Rust SDK.

Why

DEFAULT_PYTHON_VERSION is a flmexec fallback decision, not a public Rust SDK contract. Keeping it local preserves the SDK boundary while leaving FLAME_PYTHON_VERSION_ENV exported as the shared environment contract.

Validation

• cargo check -p flame-rs
• cargo test -p flmexec

[gemini-code-assist]

Code Review

This pull request moves the definition of the DEFAULT_PYTHON_VERSION constant from the Rust SDK (sdk/rust/src/lib.rs) to the local Python script execution module (flmexec/src/script/lang/python.rs). I have no feedback to provide as there are no review comments.