Add CI check for generated Python stubs

[PSU3D0]

Context

The Python stubs file (bindings/python/formualizer/__init__.pyi) is generated by cargo run --bin stub_gen and can drift from Rust bindings.

Proposal

Add a CI job that regenerates stubs and fails if there is a diff.

Acceptance criteria

• Add a CI step that runs stub generation in bindings/python
• If generated stubs differ, CI fails with a clear message
• Document local command needed to refresh stubs
• Ensure workflow works with current Python runtime setup in CI