fix(coderd/database): allow same custom role name for different orgs (#21312)

Previously the `idx_custom_roles_name_lower` index prevented that.

A check constraint was also added to ensure the `organization_id` column cannot be set to the all-zero UUID.

coderd/database/migrations/testdata/fixtures/000403_pre_allow_same_role_name_in_different_orgs.up.sql

@@ -0,0 +1,10 @@
+-- Fixture for migration 000404_allow_same_role_name_in_different_orgs.
+-- Inserts a custom role with an all-zero organization_id to ensure the
+-- migration correctly normalizes such values.
+INSERT INTO custom_roles (name, display_name, organization_id)
+VALUES (
+	'custom-role-zero-org-id',
+	'Custom Role (Zero Org ID)',
+	'00000000-0000-0000-0000-000000000000'::uuid
+)
+ON CONFLICT DO NOTHING;