Explain plan validation tests fail with non-default batch_size

DataFusion's explain plan outputs are hardcoded to expect batch_size = 8192. When batch_size is configured to different values, explain plan tests fail because the generated plans reflect the actual batch_size configuration, causing validation mismatches.

1. Document that explain plan tests are coupled to default batch_size = 8192. 2. If testing with custom batch_size values, expect explain plan test failures. 3. Update explain plan expectations when intentionally changing batch_size defaults. 4. Consider implementing batch_size-agnostic explain plan validation for better test coverage.