Fix ONLY_FULL_GROUP_BY violations in getCount() and Legacy GROUP BY tests

[opengeek]

What changed and why

Closes #190. MySQL 8.0 enforces ONLY_FULL_GROUP_BY by default. Two pre-existing violations caused all MySQL CI jobs to fail after upgrading the CI service image to mysql:8.0 (PR #277).

Files and methods changed

• src/xPDO/xPDO.php — xPDO::getCount(): replaced $query->select($expr) with $query->select('1') so the subquery inner SELECT is always GROUP BY-compliant.
• test/xPDO/Legacy/Om/xPDOQueryHavingTest.php — testHavingWithGroupBy(): added missing id and color columns to GROUP BY to match the implicit SELECT *.
• test/xPDO/Legacy/Om/xPDOQueryLimitTest.php — testLimitWithGroupBy(): added missing id and name columns to GROUP BY for the same reason.

Cross-driver impact

The getCount() fix is universal (all drivers). The test fixes are MySQL-8-triggered but correct for all drivers that enforce GROUP BY strictness.

Test coverage

SQLite suite passing: 35 tests, 35 assertions. MySQL CI will verify against mysql:8.0 in PR #277 after this merges to 3.x.

Breaking change assessment

No public API change. getCount() return value is unchanged — SELECT 1 produces the same row count as SELECT {pk} for the purpose of the outer COUNT(*). Safe for patch-level consumers.

Contributors

Reported via CI failures on PR #277.