Refactor handling of unsupported keywords in query settings fallback

This change extracts the logic for detecting unsupported keywords (like `$NOW` and `$CLUSTER_TIME`) into a separate function `containsUnsupportedKeywords()` and moves the check from `assertQuerySettingsFallback()` to the command override function. This ensures that queries containing these keywords are consistently skipped earlier in the process, as these keywords evaluate to different values at different times, making plan comparisons unreliable.