Add restriction for values of execute_mode

Now users can set any string value to `execute_mode` which is invalid and should be prevented when setting.


Look for the `execute_mode` configuration setting in the codebase, likely in a configuration validation or parsing module. The issue indicates invalid string values are currently accepted; the fix is to add validation to allow only specific valid values. Check existing tests for configuration handling to understand the expected behavior and add a test for the new restriction.

索引モデルが issue の本文から書いたものです。