[fix](nereids) do not use mv that requires union when union rewrite i…

[yx-keith]

…s disabled (#59593)

When a partitioned mv misses some queried partitions, union compensation with the base table is required to produce a complete result. The partition-compensation branch was gated by enable_materialized_view_union_rewrite, so when that flag was false the whole branch was skipped and the partial mv-only plan was used as the rewrite result, silently dropping the rows of the uncovered partitions.

Compute the invalid partitions regardless of the flag, and bail out (the mv can not be used for this query) when the mv actually needs union compensation but union rewrite is disabled - same handling as when union compensation can not be done because of the query structInfo. When the mv covers all queried partitions (no union needed) the mv is still used as before, so the flag only affects mvs that genuinely require union.

Add a regression test that, with the flag off and an mv missing a base-table partition, asserts the mv is not chosen and the query still returns the complete result.

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