fix: resolve PyO3 extension-module linking for cargo test and maturin builds

[pratyush618]

Problem

Two interacting issues caused builds to break:

1. CI rust-test — cargo test --workspace failed with undefined Py_* symbols because extension-module was always-on in workspace pyo3 features, telling PyO3 not to link libpython. Test executables need libpython linked.

2. Publish workflow (maturin builds) — After fixing (1) by removing extension-module from workspace defaults, maturin builds broke. Maturin specially handles "pyo3/extension-module" in pyproject.toml features by stripping it from --features and setting PYO3_BUILD_EXTENSION_MODULE=1 via env instead. Without extension-module in workspace defaults, the cargo feature was never activated and pyo3 tried to link libpython (cannot find -lpython3.9).

Fix

• Cargo.toml — Remove extension-module from workspace pyo3 features
• crates/taskito-python/Cargo.toml — Add extension-module = ["pyo3/extension-module"] as an explicit crate feature
• pyproject.toml — Change "pyo3/extension-module" → "extension-module" in maturin features so maturin passes it as a normal cargo feature (activating pyo3/extension-module through the crate feature, not via env var)
• .github/workflows/ci.yml — Add Python setup + LD_LIBRARY_PATH to rust-test so cargo test --workspace can link libpython

Result: maturin builds activate pyo3/extension-module (no libpython linking, correct for .so). cargo test runs without that feature and links libpython normally.