Unknown sequence in DEFAULT raises 42601 instead of 42704

[EnRaiha]

Summary

Unknown sequence in a DEFAULT expression raises SQLSTATE 42601
(plan error) today. PostgreSQL uses 42704 (undefined_object) for
nextval('missing'); the code already carries the message naming the
missing sequence, only the class is generic.

Scope

Five-file promotion path identified: error types variant, sequence_default
convert site, pgwire + gateway error maps, DDL dispatch check (~20 lines).

Related: #294 (tracked as out-of-scope candidate), #303

Reproduction

CREATE SEQUENCE s;
CREATE COLLECTION t (id BIGINT DEFAULT nextval('missing'), v TEXT) WITH (engine = 'kv');
INSERT INTO t (v) VALUES ('x');   -- SQLSTATE 42601 today; PostgreSQL: 42704

[farhan-syah]

Fixed on the branch for PR #316, verified.

CREATE COLLECTION t (k TEXT PRIMARY KEY, id BIGINT DEFAULT nextval('missing'), v TEXT) WITH (engine = 'kv');
INSERT INTO t (k, v) VALUES ('k1', 'x');

Raises ERROR: sequence "missing" does not exist (SQLSTATE 42704).

The DEFAULT path resolves the accessor through SqlCatalog against the sequence registry, and an unknown sequence name maps to undefined_object rather than the generic plan error.