Fix qutrit mixed sampling of hamiltonians

[astralcai]

Context:
TestHamiltonianSamples fails in ham-tests

Description of the Change:
Update qml.Hamiltonian to qml.ops.Hamiltonian in isinstance checks such that it always refer to the old Hamiltonian

[sc-59163]

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[PietropaoloFrisoni]

Probably I'm missing something, but I thought that this wasn't necessary anymore after this change

[astralcai]

this is to fix intermittent failures due to random fluctuations, actually unrelated to the actual failing test cases that this PR addresses

[PietropaoloFrisoni]

Ok, then it is the same issues that affected other PRs as well with opmath enabled

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.46%. Comparing base (4b35c14) to head (924d4f5).

Additional details and impacted files

@@            Coverage Diff             @@
##           ham-tests    #5413   +/-   ##
==========================================
  Coverage      99.46%   99.46%           
==========================================
  Files            400      400           
  Lines          37133    37133           
==========================================
  Hits           36935    36935           
  Misses           198      198           

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[PietropaoloFrisoni]

Thanks! LGTM