fix: add schema qualifier to cross-schema foreign key references (#94)

[tianzhou]

Fix #94

When dumping a schema with foreign keys that reference tables in other schemas (e.g., tenant_schema.table → public.table), the REFERENCES clause was missing the schema qualifier. This caused:

1. Incorrect SQL output that depends on search_path
2. pgschema plan continuously generating plans to replace the constraint

Root cause: generateConstraintSQL() ignored the targetSchema parameter and only used constraint.ReferencedTable without checking constraint.ReferencedSchema.

Fix: Use ir.QualifyEntityNameWithQuotes() to add schema qualifier when the referenced table is in a different schema than the target schema.

Enhanced testdata/dump/tenant/ test case to reproduce and prevent regression by adding a cross-schema FK from tenant schema to public.categories table.

🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

This PR fixes a bug where foreign key constraints referencing tables in different schemas were generated without schema qualifiers, causing incorrect SQL output and continuous plan generation. The fix ensures that cross-schema foreign key references include the proper schema qualifier (e.g., public.categories instead of just categories).

Key Changes:

• Modified generateConstraintSQL() to use the targetSchema parameter and qualify referenced table names when they exist in different schemas
• Added test case demonstrating cross-schema foreign key from tenant schema to public.categories table

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
testdata/dump/tenant/public.sql	Adds shared categories table in public schema for cross-schema FK testing
testdata/dump/tenant/pgschema.sql	Adds category_id column and cross-schema FK constraint to posts table
internal/diff/constraint.go	Implements schema qualification logic for foreign key references

---

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