fix(catalog): store reg* oid-alias columns as their display name, not the oid

[sunng87]

Summary

PostgreSQL reg* columns (regproc, regtype, regclass, ...) display as the object name, not the integer oid — e.g. pg_type.typinput for the text type shows textin. Commit 0300310 had exported these as ::oid integers so the oid-coercion analyzer rule could resolve name/numeric string comparisons. That benefit is now mostly gone (the coercion analyzer was replaced by SQL rewrites in #380, and the implicit comparison it served is blacklisted), while the display regression it introduced remains: clients saw the raw oid where real Postgres returns the name (part of #384).

This reverts the reg* storage to the display name (string), keeping the pg.oid_alias=<kind> field metadata so the wire layer can still report the correct alias type. Genuine oid columns stay int32 (Postgres displays oid as the integer).

Changes

• export_pg_catalog_arrow.sh (generates the committed feathers):
  • reg* family → pa.string(); dropped the ::oid SELECT cast in select_expr.
  • oid stays pa.int32(); pg.oid_alias metadata stamping unchanged.
  • POSTGRES_PORT is now env-overridable (the host-side publish is only for ad-hoc inspection; the export runs in-container).
• pg_to_arrow.py (generic exporter): it already stored reg* as string (the OIDs fall through to the pa.string() default); only the now-stale docstrings claimed int32 — corrected.
• Regenerated 10 feathers against the same postgres:17.6 (no version drift): pg_type, pg_proc, pg_aggregate, pg_operator, pg_am, pg_amproc, pg_conversion, pg_range, pg_ts_parser, pg_ts_template. Verified e.g. pg_type.typinput/typoutput/typreceive/typsend are now string + pg.oid_alias=regproc holding the name (textin, textout, ...); oid/typnamespace remain int32.
• New end-to-end test pg_type_regproc_columns_display_as_names: selecting typinput/typoutput for oid = 25 returns textin/textout.

Verification

• cargo test -p datafusion-pg-catalog → 46 passed (incl. the new test).

Trade-off

Numeric oid equality on reg* columns (WHERE typinput = 42) and an explicit col::regproc cast on a (now string) reg* column no longer take the int path. The common display/name-comparison forms — SELECT typinput, typinput::text, WHERE typinput = 'textin' — are restored.

Refs: #384