[CALCITE-7579] REGR_COUNT arguments are dropped in RelNode

[chloe-zh]

REGR_COUNT(y, x) was incorrectly converted to REGR_COUNT(*) because SqlRegrCountAggFunction extends SqlCountAggFunction, which:

1. Hardcodes SqlKind.COUNT in its constructor, causing REGR_COUNT to report the wrong SqlKind.
2. Causes RexBuilder.addAggCall() to match REGR_COUNT via SqlKind.COUNT and strip non-nullable arguments (an optimization valid only for COUNT).
3. Masked a missing case handler in AggregateReduceFunctionsRule, since SqlKind.COUNT is not in functionsToReduce.

Fix:

• Add a protected constructor to SqlCountAggFunction that accepts SqlKind, allowing subclasses to specify their own kind.
• Update SqlRegrCountAggFunction to pass SqlKind.REGR_COUNT.
• Guard the nullable-args optimization in RexBuilder.addAggCall() to check SqlKind == COUNT instead of instanceof.
• Add case REGR_COUNT in AggregateReduceFunctionsRule to preserve it as-is, since it is irreducible.

Jira Link

https://issues.apache.org/jira/browse/CALCITE-7579

Problem

When parses REGR_COUNT(y, x) the SqlKind is mistakenly set to SqlKind.COUNT instead of SqlKind.REGR_COUNT, the agg builder then drop the args y and x and replace with *, leading to empty arg list of the agg call at RelNode stage.
A way to repro it is, parse and plan it, then convert it back to SQL, it would be:

REGR_COUNT(y, x) -> REGR_COUNT(*)

which is semantically incorrect

Changes Proposed

• Correct the SqlKind to REGR_COUNT
• At build agg call, check SqlKind instead of instanceof thus it always focuses on real count and bypass regr_count
• Remove regr_count from AggregateReduceFunctionsRule given it is irreducible.

[mihaibudiu]

Before submitting a PR please file an issue using https://issues.apache.org/jira

[chloe-zh]

don't have account yet, just signed it up, waiting for update from the apache maintainer @mihaibudiu

[xiedeyantu]

I have approved your Jira account application. You can now create a Jira and assign it to yourself.

[chloe-zh]

thanks team, updated

[xiedeyantu]

The following three items(Jira Title, PR Title, Commit Message) MUST match exactly in wording and meaning, the title cannot contain the word "fix".

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mihaibudiu]

You need to add at least one unit test which would prevent this from happening again in the future.
Ideally the test which motivated you to open the issue in the first place.
The final commit message will have to match the JIRA title exactly, including [CALCITE-7579], please see the other commits.