migrate --stop-before an already-applied migration is now an error

The CLI validated --stop-before names against both pending and applied
migrations, but Migrations.apply() only looked for the stop name among
pending ones - naming an applied migration passed validation and then
silently applied every migration after it, the exact outcome the option
exists to prevent. apply() now raises ValueError before applying
anything if a stop_before name matches an applied migration in its set;
names not in the set are still ignored, since unqualified CLI values are
offered to every set. The migrate command reports it as a clean error.

Refs https://github.com/simonw/sqlite-utils/issues/769#issuecomment-4900034150

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_cli_migrate.py

@@ -463,3 +463,25 @@ def test_list_does_not_upgrade_legacy_migrations_table(two_migrations):
     db2 = sqlite_utils.Database(db_path)
     assert db2["_sqlite_migrations"].pks == ["migration_set", "name"]
     db2.close()
+
+
+def test_stop_before_applied_migration_errors(two_migrations):
+    path, _ = two_migrations
+    db_path = str(path / "test.db")
+    migrations_path = str(path / "foo" / "migrations.py")
+    # Apply everything first
+    first = CliRunner().invoke(
+        sqlite_utils.cli.cli,
+        ["migrate", db_path, migrations_path, "--stop-before", "bar"],
+    )
+    assert first.exit_code == 0
+    # foo is now applied - stopping before it is an error, and bar
+    # must not be applied as a side effect
+    result = CliRunner().invoke(
+        sqlite_utils.cli.cli,
+        ["migrate", db_path, migrations_path, "--stop-before", "foo"],
+    )
+    assert result.exit_code != 0
+    assert "already been applied" in result.output
+    db = sqlite_utils.Database(db_path)
+    assert not db["bar"].exists()