feat(mysql): implement columnStats using INFORMATION_SCHEMA.STATISTICS (#233)

* feat(mysql): implement columnStats using INFORMATION_SCHEMA.STATISTICS

Enable column cardinality statistics for the MySQL connector by querying
INFORMATION_SCHEMA.STATISTICS, which provides index-based cardinality
estimates without requiring additional permissions.

- Add generateColumnStatisticsQuery() to KtxMysqlDialect
- Add getColumnStatistics() and columnStats() to KtxMysqlScanConnector
- Flip columnStats capability from false to true
- Add MysqlStatsRow and KtxMysqlColumnStatisticsResult interfaces
- Add tests for dialect query generation and connector stats retrieval
- Update dialect conformance fixture for mysql

* fix(mysql): filter to leading index columns to avoid inflated cardinality

Add AND SEQ_IN_INDEX = 1 to INFORMATION_SCHEMA.STATISTICS query to
ensure only leading index columns are returned. For composite indexes,
non-leading columns report the cardinality of the index prefix rather
than the column's own distinct count, which inflates distinctCount.

Add regression test asserting SEQ_IN_INDEX = 1 is present in the query.

* fix: add trailing newline to dialect.test.ts

---------

Co-authored-by: Andrey Avtomonov <andreybavt@gmail.com>

packages/cli/test/context/connections/dialects.test.ts

@@ -89,7 +89,7 @@ const fixtures: DialectFixture[] = [
     cardinalityContains: 'SELECT COUNT(DISTINCT val) AS cardinality',
     randomizedCardinalityContains: 'ORDER BY RAND()',
     distinctValuesContains: 'SELECT DISTINCT CAST(`status` AS CHAR) AS val',
-    statisticsContains: null,
+    statisticsContains: 'INFORMATION_SCHEMA.STATISTICS',
     dimensionInput: 'tinyint(1)',
     dimensionType: 'boolean',
     nativeTypeInput: 'varchar(255)',