fix(config): edge cases in models.yaml migration

When running the configurator, the `legacy_models_conf_path` was stripped when saving the config file. Then the migration logic didn't fire correctly, and the custom models.yaml paths weren't migrated into the db.

- Rework the logic to migrate this path by adding it to the config object as a normal field that is not excluded from serialization.
- Rearrange the models.yaml migration logic to remove the legacy path after migrating, then write the config file. This way, the legacy path doesn't stick around.
- Move the schema version into the config object.
- Back up the config file before attempting migration.
- Add tests to cover this edge case

tests/test_config.py

@@ -9,16 +9,14 @@ from pydantic import ValidationError
 from invokeai.app.services.config.config_default import InvokeAIAppConfig, get_config, load_and_migrate_config
 
 v4_config = """
-meta:
-  schema_version: 4
+schema_version: 4
 
 host: "192.168.1.1"
 port: 8080
 """
 
 invalid_v5_config = """
-meta:
-  schema_version: 5
+schema_version: 5
 
 host: "192.168.1.1"
 port: 8080
@@ -44,6 +42,12 @@ InvokeAI:
     max_vram_cache_size: 50
 """
 
+v3_config_with_bad_values = """
+InvokeAI:
+  Web Server:
+    port: "ice cream"
+"""
+
 invalid_config = """
 i like turtles
 """
@@ -88,6 +92,29 @@ def test_migrate_v3_config_from_file(tmp_path: Path):
     assert not hasattr(config, "esrgan")
 
 
+def test_migrate_v3_backup(tmp_path: Path):
+    """Test the backup of the config file."""
+    temp_config_file = tmp_path / "temp_invokeai.yaml"
+    temp_config_file.write_text(v3_config)
+
+    load_and_migrate_config(temp_config_file)
+    assert temp_config_file.with_suffix(".yaml.bak").exists()
+    assert temp_config_file.with_suffix(".yaml.bak").read_text() == v3_config
+
+
+def test_failed_migrate_backup(tmp_path: Path):
+    """Test the failed migration of the config file."""
+    temp_config_file = tmp_path / "temp_invokeai.yaml"
+    temp_config_file.write_text(v3_config_with_bad_values)
+
+    with pytest.raises(RuntimeError):
+        load_and_migrate_config(temp_config_file)
+    assert temp_config_file.with_suffix(".yaml.bak").exists()
+    assert temp_config_file.with_suffix(".yaml.bak").read_text() == v3_config_with_bad_values
+    assert temp_config_file.exists()
+    assert temp_config_file.read_text() == v3_config_with_bad_values
+
+
 def test_bails_on_invalid_config(tmp_path: Path):
     """Test reading configuration from a file."""
     temp_config_file = tmp_path / "temp_invokeai.yaml"