fix: missing parentheses in CREATE POLICY USING clause output

[tianzhou]

PostgreSQL's CREATE POLICY syntax requires parentheses around USING and WITH CHECK expressions. When pg_get_expr returns simple function calls without outer parentheses (e.g., is_admin()), pgschema was outputting invalid SQL like USING is_admin() instead of USING (is_admin()).

Added ensureParentheses() helper to wrap expressions in parentheses if not already wrapped.

Fix #259

[Copilot]

Pull request overview

This PR ensures that generated CREATE POLICY / ALTER POLICY statements always use the PostgreSQL-required parenthesized syntax for USING and WITH CHECK expressions, even when pg_get_expr omits outer parentheses. It also adds file-based diff fixtures to validate the new behavior against a real-world reproduction of Issue #259.

Changes:

• Updated internal/diff/policy.go to route USING and WITH CHECK expressions through a new ensureParentheses helper so that emitted SQL uses USING (expr) / WITH CHECK (expr) consistently.
• Added an is_admin() helper function and an admin_only RLS policy in testdata/diff/create_policy/add_policy along with updated old.sql, new.sql, diff.sql, plan.sql, plan.txt, and plan.json fixtures to cover function-call policies and verify correct parentheses in generated SQL.
• Refreshed the JSON plan fingerprint and policy entries to include the newly created admin_only policy.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/diff/create_policy/add_policy/plan.txt	Adds the admin_only policy to the human-readable diff plan, ensuring the new policy appears in the expected migration steps.
testdata/diff/create_policy/add_policy/plan.sql	Updates the expected SQL plan to include CREATE POLICY admin_only ... USING (is_admin());, validating correct parenthesization in generated SQL.
testdata/diff/create_policy/add_policy/plan.json	Updates the plan JSON hash and adds a table.policy entry for public.users.admin_only, aligning structured expectations with the new policy.
testdata/diff/create_policy/add_policy/old.sql	Introduces the is_admin() function in the “old” schema to set up the pre-diff state for Issue #259.
testdata/diff/create_policy/add_policy/new.sql	Adds the admin_only RLS policy using USING (is_admin()); in the “new” schema, providing input DDL for the new regression test.
testdata/diff/create_policy/add_policy/diff.sql	Extends the expected migration SQL diff to include the CREATE POLICY admin_only ... USING (is_admin()); statement.
internal/diff/policy.go	Wraps policy.Using and policy.WithCheck with ensureParentheses in CREATE/ALTER policy generation and introduces the helper implementation, ensuring compliance with PostgreSQL’s USING (expr) / WITH CHECK (expr) syntax while preserving already-wrapped expressions.

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