test(python): load checked-in error classes for Rust tests (#4876)

Bare Rust test runners do not install the `lancedb` Python package, so
the typed-error tests fail with `ModuleNotFoundError`. Load the
checked-in exception modules once for the embedded interpreter,
preserving exception-class identity across parallel tests and the
existing type/cause assertions.

Rust CI now watches the binding sources and the Python exception modules
embedded by these tests, so those changes exercise the macOS Rust test
jobs.

.github/workflows/rust.yml

@@ -14,6 +14,9 @@ on:
       - docs/web-tests/blob-v2/rs/**
       - nodejs/Cargo.toml
       - python/Cargo.toml
+      - python/src/**
+      - python/python/lancedb/exceptions.py
+      - python/python/lancedb/remote/errors.py
       - ci/build_no_lock.py
       - ci/tests/test_build_no_lock.py
       - .github/actions/setup-protoc/**