fix(Migrator): don't let rows.Close() overwrite ColumnTypes errors

[h2zi]

ColumnTypes defers err = rows.Close() onto its named return value, unconditionally replacing whatever error occurred after the rows were opened — an error from rows.ColumnTypes() or the merge loop is swallowed by the (almost always nil) Close result, and the caller sees success with incomplete data. Keep the first error, still surfacing a Close failure when nothing else went wrong.

🤖 Generated with Claude Code

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Merge order (this batch: #241 → #242 → #243 → #244 → #245 → #246, plus #234 from the previous batch): all seven are functionally independent and merge cleanly in any order. Several append tests to the same test files, so whichever merges later may show a trivial append-only conflict in migrator_test.go/ddlmod_test.go — I'll rebase the remaining ones promptly after each merge, as before.

[Copilot]

Pull request overview

Fixes Migrator.ColumnTypes error handling so that a deferred rows.Close() does not overwrite an earlier error from rows.ColumnTypes() or the column-type merge loop, while still returning a Close error when no prior error occurred.

Changes:

• Update deferred rows.Close() handling to only set the return error when the current error is nil.
• Preserve the first error encountered during ColumnTypes execution instead of potentially masking it with a (usually nil) close result.

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