Remove incorrect assertion on "processedRows" cursor stat in a test

[azawlocki-sbdt]

Description

The test test_select_quert_stats() is flaky due to an assertion that checks that the "processedRows" cursor stat is non-decreasing across responses. In fact, the stat may decrease, especially for LIMIT queries, as such queries may involve cancelling some Trino worker tasks, and the stat is computed by aggregating over active workers (verified in Trino code).

The PR replaces the incorrect assertion with one that simply checks that "processedRows" exists in the stats.

Some cases when the assertion failed:

• https://github.com/trinodb/trino-python-client/actions/runs/29265674601/job/86870089715?pr=603#step:5:66
• https://github.com/trinodb/trino-python-client/actions/runs/27707800003/job/81960775843#step:5:64
• https://github.com/trinodb/trino-python-client/actions/runs/26816161416/job/79061743155#step:5:64.

Non-technical explanation

The PR fixes a flaky intergration test by replacing the assertion that checks a property not guaranteed by Trino by a weaker one.

Release notes

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* Fix some things. ({issue}`issuenumber`)