fix(python): preserve credential placeholder env on snapshot restore

[chaliy]

Motivation

• Snapshot restore was overwriting freshly generated credential-placeholder env vars installed by credential_placeholder(...), causing script-visible placeholders to be stale or absent while HTTP hooks expected newly generated tokens.
• This broke authenticated requests after from_snapshot()/restore_snapshot() flows without exposing the real credential, so restored instances could silently fail network auth.

Description

• Capture live placeholder env values before restoring snapshot and re-apply them after the snapshot state is loaded by adding restore_live_bash_with_env_overrides and placeholder_env_overrides helper functions in crates/bashkit-python/src/lib.rs.
• Update PyBash::restore_snapshot and BashTool::restore_snapshot to collect placeholder env overrides and call the new restore helper so placeholders remain aligned with active HTTP credential hooks.
• Add a regression test test_bashtool_from_snapshot_reapplies_placeholder_env_after_restore in crates/bashkit-python/tests/test_network_credentials.py to assert that a placeholder env is present after BashTool.from_snapshot() when the source snapshot lacks placeholder env state.

Testing

• Ran cargo fmt --all, which completed successfully.
• Ran python -m pytest crates/bashkit-python/tests/test_network_credentials.py -k from_snapshot which failed in this environment with ModuleNotFoundError: No module named 'bashkit' during test collection so the Python-level test could not be executed here.
• Invoked cargo test -p bashkit-python --no-run which began dependency resolution and compilation but could not complete within the validation window (network/dependency fetch in progress), so no final pass/fail was recorded.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	89e35b6	Commit Preview URL	May 08 2026, 09:15 AM