Make constraint queries schema-aware for multi-tenancy

Prior to this change, the ConstraintQueries class queried
pg_catalog.pg_constraint without filtering by PostgreSQL schema.
This caused incorrect behavior in multi-tenant environments using
django-tenants, where each tenant has its own schema. The queries
would detect constraints from other tenants' schemas, leading to
false positives (e.g., ConstraintAlreadyExists errors when creating
a second tenant).

This change adds INNER JOINs with pg_catalog.pg_class and
pg_catalog.pg_namespace to filter constraints by current_schema().
This ensures that constraint checks only look at the current tenant's
schema, not across all schemas in the database.

Affected queries:
- CHECK_EXISTING_CONSTRAINT
- CHECK_CONSTRAINT_IS_VALID
- CHECK_CONSTRAINT_IS_NOT_VALID

Related issue: #92

Author: RJPercival

src/django_pg_migration_tools/operations.py

@@ -53,25 +53,34 @@ class IndexQueries:
 
 class ConstraintQueries:
     CHECK_EXISTING_CONSTRAINT = dedent("""
-        SELECT conname
-        FROM pg_catalog.pg_constraint
-        WHERE conname = {constraint_name};
+        SELECT con.conname
+        FROM pg_catalog.pg_constraint con
+        INNER JOIN pg_catalog.pg_class rel ON rel.oid = con.conrelid
+        INNER JOIN pg_catalog.pg_namespace nsp ON nsp.oid = rel.relnamespace
+        WHERE con.conname = {constraint_name}
+            AND nsp.nspname = current_schema();
     """)
 
     CHECK_CONSTRAINT_IS_VALID = dedent("""
         SELECT 1
-        FROM pg_catalog.pg_constraint
+        FROM pg_catalog.pg_constraint con
+        INNER JOIN pg_catalog.pg_class rel ON rel.oid = con.conrelid
+        INNER JOIN pg_catalog.pg_namespace nsp ON nsp.oid = rel.relnamespace
         WHERE
-            conname = {constraint_name}
-            AND convalidated IS TRUE;
+            con.conname = {constraint_name}
+            AND con.convalidated IS TRUE
+            AND nsp.nspname = current_schema();
     """)
 
     CHECK_CONSTRAINT_IS_NOT_VALID = dedent("""
         SELECT 1
-        FROM pg_catalog.pg_constraint
+        FROM pg_catalog.pg_constraint con
+        INNER JOIN pg_catalog.pg_class rel ON rel.oid = con.conrelid
+        INNER JOIN pg_catalog.pg_namespace nsp ON nsp.oid = rel.relnamespace
         WHERE
-            conname = {constraint_name}
-            AND convalidated IS FALSE;
+            con.conname = {constraint_name}
+            AND con.convalidated IS FALSE
+            AND nsp.nspname = current_schema();
     """)
 
     ALTER_TABLE_CONSTRAINT_NOT_NULL_NOT_VALID = dedent("""