Horne extraction test for non-flat trace is unstable

The last `assert_quantity_allcose` in `test_horne_non_flat_trace` test checking that the Horne extraction from a flat trace equals the one from a non-flat trace fails occasionally. This seems to be rather rare and random issue, and difficult to reproduce, but it would be very good to understand whether this is a problem in the test design or Horne extraction itself. An easy fix would be to increase the assertion tolerances, but we shouldn't do this before we understand what is causing the discrepancy.

Start with the test_horne_non_flat_trace test and its final assert_quantity_allcose assertion, then inspect the Horne extraction path it exercises. Run the test repeatedly to characterize the intermittent discrepancy and determine whether it comes from the test design or extraction; done means documenting the cause and adding a reliable fix or regression coverage without merely widening tolerances.

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