Fix the failed observable serialization unit tests

[maliasadi]

PR PennyLaneAI/pennylane#5475 led to LQ and LK serialization unit tests failures. This PR updates them to pass on CIs.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.99%. Comparing base (d0e1c51) to head (44135a0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #683      +/-   ##
==========================================
- Coverage   99.37%   97.99%   -1.38%     
==========================================
  Files          51      207     +156     
  Lines        7639    29909   +22270     
==========================================
+ Hits         7591    29309   +21718     
- Misses         48      600     +552     

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[vincentmr]

If that works, no blocker from me. Thanks @maliasadi !

[multiphaseCFD]

Thanks @maliasadi ! Do we also want to turn on mgpu tests?

[mudit2812]

Device tests are still failing with LGPU and LK. Let's figure out if the issue is on the lightning side or PL side before merging this.

[tomlqc]

[sc-61588]