## Code Analysis

### Generated marker is populated, but the expression field is not
`server/tables/information_schema/columns_table.go:131-145`
```go
func getRowFromColumn(ctx *sql.Context, curOrdPos int, col *sql.Column, catName, schName, tblName string) sql.Row {
	var (
		ordinalPos  = int32(curOrdPos + 1)
		nullable    = "NO"
		isGenerated = "NEVER"
	)

	if col.Generated != nil {
		isGenerated = "ALWAYS"
	}
```
The row builder correctly maps a non-nil `col.Generated` value to `is_generated = ALWAYS`.

`server/tables/information_schema/columns_table.go:187-197`
```go
		"NO",                  // is_self_referencing TODO
		"NO",                  // is_identity TODO
		nil,                   // identity_generation TODO
		nil,                   // identity_start TODO
		nil,                   // identity_increment TODO
		nil,                   // identity_maximum TODO
		nil,                   // identity_minimum TODO
		"NO",                  // identity_cycle TODO
		isGenerated,           // is_generated
		nil,                   // generation_expression TODO
		"YES",                 // is_updatable
```
The same row always supplies `nil` for `generation_expression`, so a generated column's expression cannot be returned through `information_schema.columns`. This source path directly explains the observed blank value while preserving the correct generated marker.

### Observed catalog readback
The test created the generated table, added the regular column, and queried both catalog views. The command completed successfully.

```text
# REV-4 generated_catalog metadata regression
DROP TABLE
CREATE TABLE
ALTER TABLE
# information_schema.columns
 column_name | is_generated | generation_expression 
-------------+--------------+-----------------------
 id          | NEVER        | 
 source      | NEVER        | 
 result      | ALWAYS       | 
 extra       | NEVER        | 
(4 rows)

# pg_attribute
 column_name | attgenerated 
-------------+--------------
 id          | 
 source      | 
 result      | s
 extra       | 
(4 rows)

# psql_exit=0
```

### Result
The runtime readback confirms that `generated_catalog.result` is recognized as generated (`ALWAYS`, `attgenerated = s`) but has an empty `generation_expression`. The source row construction contains the corresponding unconditional `nil`, supporting the reported metadata defect.

### Test context
The captured test output reports no mocks, bypasses, or forced faults. The available evidence includes the catalog readback and source-backed causal analysis; it does not include the original SQL command text.