PATCH /columns rewrites GENERATED ALWAYS identity to BY DEFAULT when identity_generation is omitted

[hsusul]

Describe the bug

PATCH /columns/:id (and PostgresMeta.columns.update) rewrites a GENERATED ALWAYS identity column to GENERATED BY DEFAULT whenever identity_generation is omitted.

Updating only a comment, name, nullability, or type therefore changes identity generation as a side effect.

To Reproduce

const created = await pgMeta.columns.create({
  table_id,
  name: 'id',
  type: 'int8',
  is_identity: true,
  identity_generation: 'ALWAYS',
})

const updated = await pgMeta.columns.update(created.data.id, { comment: 'pk' })
// updated.data.identity_generation === 'BY DEFAULT'

Expected behavior

Omitting identity_generation leaves generation unchanged (ALWAYS stays ALWAYS). That is what the state table in PostgresMetaColumns.update already documents: when the column is already identity and identity_generation is undefined, emit no SET GENERATED.

Actual behavior

The parameter default identity_generation = 'BY DEFAULT' makes the === undefined branch unreachable, so every update of an identity column emits SET GENERATED BY DEFAULT.

Additional context

POST /columns should keep defaulting omitted identity_generation to BY DEFAULT (that was #77). The update-path default is the bug.

• Library: @supabase/postgres-meta
• Branch analyzed: master (641831e)