Add regression/db-connection-leak-valgrind

[sergio-correia]

This test uses valgrind and is based on the existing test
regression/db-connection-leak-reproducer.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

This pull request currently contains no functional or content changes; it appears to be a placeholder or test PR with an empty diff for plans/main.fmf.

File-Level Changes

Change	Details	Files
No effective changes were made; the diff is empty and the file content is unchanged.	File is referenced in the diff header but has no added, removed, or modified lines. There is currently nothing to validate functionally or logically in the codebase for this PR.	plans/main.fmf

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[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

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[kkaarreell]

I have just a few nitpicks.
However, it seems that the test reports various connection leaks even for the current upstream code and not consistently.