test: add generated-column fixtures and fix AT_SetExpression/AT_DropExpression deparsing

[pyramation]

Summary

Add comprehensive fixture coverage for generated columns and fix the AlterTableCmd deparsing for AT_SetExpression and AT_DropExpression.

What changed

• Added __fixtures__/kitchen-sink/misc/generated-columns.sql with round-trip statements for:
  • CREATE TABLE ... GENERATED ALWAYS AS (expr) STORED (arithmetic, function calls, type modifiers, CASE, NOT NULL/UNIQUE)
  • ALTER TABLE ... ADD COLUMN ... GENERATED ALWAYS AS (expr) STORED
  • ALTER TABLE ... ALTER COLUMN ... SET EXPRESSION AS (expr)
  • ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION
  • ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION IF EXISTS
• Regenerated __fixtures__/generated/generated.json and packages/deparser/__tests__/kitchen-sink/misc-generated-columns.test.ts.
• Fixed packages/deparser/src/deparser.ts AlterTableCmd:
  • AT_SetExpression now emits SET EXPRESSION AS (expr) so it round-trips.
  • AT_DropExpression now emits DROP EXPRESSION IF EXISTS when missing_ok is true.

Tests

• npx jest __tests__/kitchen-sink/misc-generated-columns.test.ts passes.
• Full npx jest in packages/deparser passes: 292 suites, 712 tests, 424 snapshots.

Link to Devin session: https://app.devin.ai/sessions/71d24cab0da24e7b83d3f5a6e9fef9a2
Requested by: @pyramation

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring