fix: generate_column_not_null

[tianzhou]

Fix #86

[Copilot]

Pull Request Overview

This PR fixes issue #86 by allowing NOT NULL constraints to be applied to generated columns in PostgreSQL schema generation. The fix modifies the column clause building logic to remove the exclusion of generated columns from NOT NULL constraint generation.

• Removes the condition that prevented NOT NULL constraints on generated columns
• Updates test data to include a new generated column with NOT NULL constraint

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/diff/table.go	Modified buildColumnClauses to allow NOT NULL on generated columns
testdata/diff/create_table/add_column_generated/plan.txt	Added test case for generated column with NOT NULL
testdata/diff/create_table/add_column_generated/plan.sql	Added SQL for generated column with NOT NULL
testdata/diff/create_table/add_column_generated/plan.json	Added JSON plan entry for generated column with NOT NULL
testdata/diff/create_table/add_column_generated/new.sql	Added generated column definition with NOT NULL
testdata/diff/create_table/add_column_generated/diff.sql	Added ALTER statement for generated column with NOT NULL

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}