sync_diff_inspector: check rows.Err in TiDB iterator

[banerjeeadrish]

What problem does this PR solve?

Issue Number: close #900

sync-diff-inspector can silently swallow TiDB row-stream errors during row comparison.

When TiDBRowsIterator.Next() sees rows.Next() == false, it currently returns (nil, nil) directly. However, per the database/sql contract, rows.Next() == false can mean either clean EOF or an iteration error. The caller must check rows.Err() to distinguish the two cases.

If the TiDB-side query is cancelled mid-stream, for example due to tidb_mem_quota_query, sync-diff-inspector can incorrectly treat the TiDB iterator as exhausted. This may produce false +N/-0 diffs and bogus REPLACE INTO fix SQL instead of surfacing the original TiDB query error.

What is changed and how it works?

This PR updates TiDBRowsIterator.Next() to check s.rows.Err() after s.rows.Next() returns false.

If rows.Err() is non-nil, the iterator now returns the error instead of treating it as clean EOF. The existing row comparison flow already propagates non-nil iterator errors, so the table comparison fails with the real TiDB error instead of generating false fix SQL.

A unit test was added to cover the row-stream error case and verify that TiDBRowsIterator.Next() returns the underlying database/sql error.

Check List

Tests

• Unit test

Code changes

• None

Side effects

• None

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

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