crosscluster/logical: transactional LDR REGIONAL BY ROW implicit partition columns produce wrong unique lock IDs

[jeffswenson]

Describe the problem

CollectKeyColumnIDs on an implicitly partitioned unique index includes
crdb_region, but uniqueness is enforced across regions — two txns in different
regions taking the same unique value get different lock hashes.

Impact

Missed conflicts on REGIONAL BY ROW unique indexes. Reachability uncertain (is
RBR intended to be supported for txn LDR?); if not, it needs an explicit
validation gate rather than silent mis-locking.

Code references

• txnlock/schema.go:62

Suggested fix

If RBR is unsupported, add an explicit validation gate at txn-mode creation. If
supported, exclude implicit partition columns from unique lock derivation.

Epic CRDB-65552

Jira issue: CRDB-65611

[blathers-crl]

Hi @jeffswenson, please add a branch-* label to identify the earliest affected branch for this C-bug

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[blathers-crl]

cc @cockroachdb/cdc