Fix v2 finalization rollback on duplicate tx

[raymondjacobson]

Summary

• add a regression for an accepted block containing the same v2 ERN transaction twice
• isolate v2 transaction finalization in a nested pg savepoint so a tx-level finalization error rolls back only that tx side effect work
• preserve the existing behavior where the duplicate tx returns Code=2, but the block transaction can still commit

Test plan

• TEST_DB_URL=postgres://localhost:55438/openaudio_test?host=/tmp/openaudio-hunt4-pg.pYbDYu go test ./pkg/core/server -run=TestFinalizeBlockDuplicateV2ERNTransactionDoesNotPoisonCommit -count=1 -v
• TEST_DB_URL=postgres://localhost:55438/openaudio_test?host=/tmp/openaudio-hunt4-pg.pYbDYu go test ./pkg/core/server -count=1