Optimize column_encryption_policy checks in recv_results_rows

[mykaul]

There's no point in checking a global policy for every single value decoding, not for every row decoded.

Adjusted the code to only check it once per recv_results_rows() call - decode_row() should be defined either as is today with column_encryption_policy enabled,
or much simpler without all those extra checks.

Added a unit test from CoPilot.

Fixes: #582

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[mykaul]

CI failure is unrelated, and I've seen it happening on too many PRs already :-/