fix: improve delete_metrics_splits error handling and doc comments (#6280)

* review: parquet_file singular, proto doc link, fix metastore accessor

* style: fix rustfmt nightly comment wrapping in split metadata

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: use plain code span for proto reference to avoid broken rustdoc link

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>