fix: update clickbench expected plan for NDV-aware optimization

[asolimando]

Which issue does this PR close?

Fixes CI breakage on main introduced by #19957.

Rationale for this change

#19957 introduced NDV extraction from Parquet metadata. The optimizer now sees NDV=1 for HitColor, BrowserCountry, BrowserLanguage in the clickbench test file and short-circuits COUNT(DISTINCT) to a constant projection, skipping the full table scan.

What changes are included in this PR?

Updates the expected EXPLAIN plan in clickbench.slt to match the new (better) physical plan:

-   01)AggregateExec: mode=Single, gby=[], aggr=[count(DISTINCT hits.HitColor), ...]
-   02)--DataSourceExec: file_groups={1 group: [...]}, projection=[HitColor, BrowserLanguage, BrowserCountry], file_type=parquet
+   01)ProjectionExec: expr=[1 as count(DISTINCT hits.HitColor), 1 as count(DISTINCT hits.BrowserCountry), 1 as count(DISTINCT hits.BrowserLanguage)]
+   02)--PlaceholderRowExec

Are these changes tested?

This PR is the test fix. Verified locally with cargo test --profile ci -p datafusion-sqllogictest --test sqllogictests.

Are there any user-facing changes?

No.

[asolimando]

(cc: @timsaucer as you reported the problem in CI and you have rights to merge the PR if it looks OK to you, tested locally but I might have missed something, waiting for CI)

[timsaucer]

Thank you!