storage: version the PostgreSQL oid source cast per export

[def-]

Follow-up to #37440 Sorry for only realizing so late that it's an issue after all! Without this PR bin/mzcompose --find platform-checks run default --scenario=UpgradeEntireMz --check=PgCdcOidRetraction fails like this after the upgrade:

> SELECT id, o FROM oid_retraction_table ORDER BY id;
2026-07-06T13:19:56.907798Z  WARN mz_testdrive::action::consistency: No Catalog state on disk, skipping consistency check
^^^ +++
2:1: error: executing query failed: db error: ERROR: Source error: source must be dropped and recreated due to failure: failed to cast raw column: invalid input syntax for type oid: number too large to fit in target type: "4294967295"

Since the source is not an errored state it's possible customers could run into this.

parse_oid was widened to the full u32 range (#37440), but the same helper backs CastStringToOid in the PostgreSQL source, whose eval behavior must stay stable across releases. Replication re-casts the old tuple on delete, so a value ingested pre-upgrade as a CastError would be retracted post-upgrade as a value. That leaves the error stuck and adds a phantom negative row.

The fix versions the cast per export:

• CastStringToOid is kept on a frozen parse_oid_legacy that accepts only the i32 range, so exports created before the widening keep retracting cleanly across upgrades. The SQL, COPY, and pgwire paths keep the widened parse_oid.
• Newly purified exports persist a cast_oid_full_range flag in their statement DETAILS and plan a new CastStringToOidFullRange variant that accepts the full u32 range, so fresh sources ingest valid oid values above i32::MAX. Column casts are regenerated from the persisted details on every plan, so deriving the choice from the details (rather than the binary version) keeps it stable for the lifetime of an export.

Tests: a unit snapshot pinning the frozen legacy behavior, error snapshot and SQL-cast parity tests for the full-range variant, a platform-check reproducing the cross-upgrade retraction, and a pg-cdc testdrive file covering snapshot, replication, and retraction of above-i32::MAX oids in scalar and array columns.