Removes commented-out ProviderMetrics tests (struct was replaced by
ConnectionStatus long ago) and reword the Parquet-reader integration
tests so they stop claiming the reader is a "placeholder" — the Parquet
reader is fully implemented and surfaces `File::open` errors via
anyhow. Also tightens test_12_invalid_file_handling to assert the real
Err behaviour rather than the stale "Ok(vec![])" expectation.
No production code change; tests still compile and run under the same
ignore gates.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>