Add max-union-size-per-datasource Property

[wy1433]

Fixes #23635.

Changes proposed in this pull request:

• Add max-union-size-per-datasource property to control UNION ALL merge behavior
• When route units count for a datasource exceeds the threshold, split into batches to restore parallel execution capability
• Default value is Integer.MAX_VALUE to preserve existing behavior
• Skip aggregate rewrite when max-union-size-per-datasource = 1 for better performance

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[terrymanu]

Thanks for the PR! Code looks good—max-union-size-per-datasource keeps default behavior, addresses the same-database UNION ALL long-SQL timeout case, and docs/tests are updated. No side effects spotted, so I’m ready to merge. Thanks again and welcome more contributions!