Fix REGR_COUNT arguments being dropped during SqlToRel conversion

[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

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