fix: properly drop temporary NOT NULL constraint

[sam-mosleh]

Problem Statement

When adding and removing NOT NULL constraints, pgschema fails to properly clean up temporary constraints. This leads to constraint name collisions and migration failures when attempting to re-add the constraint.

Error Message

ERROR: constraint "age_not_null" for relation "users" already exists (SQLSTATE 42710)

Root Cause

pgschema creates named constraints when NOT NULL is added to a column. When the constraint is removed in a subsequent migration, the temporary constraint object is not being properly dropped from the database. This orphaned constraint prevents future migrations from creating a new constraint with the same name.

Reproduction Steps

The issue can be reproduced with the following sequence of schema changes:

-- Step #1: Create table without constraint
CREATE TABLE users (
    id int PRIMARY KEY,
    age int
);

-- Step #2: Add NOT NULL constraint
CREATE TABLE users (
    id int PRIMARY KEY,
    age int NOT NULL
);

-- Step #3: Remove NOT NULL constraint
CREATE TABLE users (
    id int PRIMARY KEY,
    age int
);

-- Step #4: Re-add NOT NULL constraint (FAILS)
CREATE TABLE users (
    id int PRIMARY KEY,
    age int NOT NULL
);

Expected Result: Step 4 should succeed
Actual Result: Error thrown due to constraint name collision

[Copilot]

Pull Request Overview

This PR fixes a bug where temporary CHECK constraints used during NOT NULL column migrations were not being properly cleaned up, causing constraint name collisions on subsequent migrations.

Key Changes:

• Added a 4th step to the NOT NULL migration workflow that drops the temporary CHECK constraint after setting the column to NOT NULL
• Updated test data to reflect the new cleanup step

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
internal/plan/rewrite.go	Added logic to generate and execute DROP CONSTRAINT statement for temporary CHECK constraint cleanup
testdata/diff/online/add_not_null/plan.txt	Updated expected migration plan output to include constraint cleanup
testdata/diff/online/add_not_null/plan.sql	Updated expected SQL output to include DROP CONSTRAINT statement
testdata/diff/online/add_not_null/plan.json	Updated expected JSON plan to include new cleanup step

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[tianzhou]

LGTM. Thanks for the fix.