fix(python-guest): persist module globals across run() calls

[simongdavies]

The Executor previously rebuilt the globals dict on every call to run() via an inline 'exec(code, {...})' literal. That silently discarded every 'def', 'class', and top-level assignment between runs on the same sandbox instance, breaking three documented contracts:

Fix: construct the globals dict once in Executor.init and pass the instance attribute to every exec(). Snapshot/restore continues to rewind the namespace because it lives in the guest's Wasm linear memory.

Adds tests/python_state_persistence.rs covering: top-level def reuse, bare assignment reuse, and snapshot/restore rewind of the persistent namespace.

[Copilot]

Pull request overview

This PR fixes the Python Wasm guest executor so that module-level globals persist across multiple run() calls on the same sandbox instance (aligning behavior with the documented snapshot/restore semantics and the existing examples). It also adds an integration test suite to prevent regressions.

Changes:

• Persist Python guest module globals by constructing the exec() globals dict once in Executor.__init__ and reusing it on every run().
• Add integration tests validating (1) top-level def reuse, (2) top-level assignment reuse, and (3) snapshot/restore rewinding of the persistent namespace.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/wasm_sandbox/guests/python/sandbox_executor.py	Reuses a single self._globals dict across run() calls instead of recreating it each time.
src/wasm_sandbox/tests/python_state_persistence.rs	Adds integration coverage for Python state persistence across runs and correct snapshot/restore rollback behavior.