fix(ui): cache total count once across CSV export pagination (#40232)

[lbajsarowicz]

Description

ConvertToCsv::getCsvFile() called $searchResult->setTotalCount($totalCount)
on the search result after getItems(). DataProvider::getSearchResult()
delegates to Reporting::search() which builds a fresh collection per call,
so the cached count was discarded between iterations and each page paid
another getSelectCountSql() round trip. With pageSize=1 this scales to
N count queries for an N-row export.

Computed total count once before the loop, reused the first search result
for iteration 1, and seeded the cached total on every subsequent fresh
result via setTotalCount() so downstream getTotalCount() callers skip
the redundant aggregate query.

Fixes #40232

Fixed Issues

• Fixes Slow performance when exporting CSV from the UI listing component with large dataset #40232: Slow performance when exporting CSV from the UI listing component with large dataset

Manual testing scenarios

1. Place 10 orders.
2. Temporarily reduce $pageSize in Magento\Ui\Model\Export\ConvertToCsv::__construct to 1 to amplify the issue.
3. Add SLEEP(1) to the order grid count query (or profile with Xdebug/SPX).
4. Export Sales > Orders as CSV.
5. After this fix: a single getSelectCountSql call. Before: one per page.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (18 unit tests pass)
• Changes to the codebase comply with technical guidelines

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[lbajsarowicz]

@magento run all tests