Share the native-reader admission checks between the core Parquet scan and the Delta contrib #5944

What is the problem the feature request solves?

DeltaScanSupport in contrib/delta-spark repeats three admission checks that core's native Parquet scan already performs: the input-file-expression check, the vectorized-reader-setting check, and the nested-default check. A future fix to any of them needs a separate Delta update.

Describe the potential solution

A shared native-reader admission helper in core that both scans call, leaving Delta's protocol checks, deletion-vector row-index exception, liveness checks and stricter encryption fallback explicit in the contrib. Filesystem policy stays with #5658. A parity test covering the common rejection cases for ordinary Parquet and Delta, plus supported-scan controls, protects the boundary.

Additional context

Raised in the #5365 review (item 2); filed as a follow-up so the refactor of the shared core path gets its own review.