[FEATURE] Add Integration tests for `explain` with pushdown disabled in Calcite

[yuancu]

Is your feature request related to a problem?

Currently, all integration tests for the EXPLAIN command are limited to scenarios where pushdown is enabled. For example, in ExplainIT.java:

String expected =
    isCalciteEnabled()
        ? loadFromFile("expectedOutput/calcite/explain_output.json")
        : loadFromFile("expectedOutput/ppl/explain_output.json");

Both cases rely on execution with pushdown enabled, leaving a gap in test coverage.

What solution would you like?
Add new EXPLAIN tests that explicitly run with pushdown disabled, to verify the correctness and completeness of the explain output when fallback to Calcite occurs.

What alternatives have you considered?

Do you have any additional context?
#3777