NullQubit tests with new graph-based decomposition

[JerryChen97]

Context:
NullQubit has a new test that exclusively works for new graph-based decomposition system.

Description of the Change:
Some previous tests have been used with the dual mode tests with or without graph decomp
A new graph exclusive test added, mimicingg the one in DQ tests.

Benefits:

Possible Drawbacks:

Related GitHub Issues:
[sc-97952] #8286

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.39%. Comparing base (2e4c60f) to head (7303c29).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8260   +/-   ##
=======================================
  Coverage   99.39%   99.39%           
=======================================
  Files         558      558           
  Lines       57781    57786    +5     
=======================================
+ Hits        57431    57436    +5     
  Misses        350      350           

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[albi3ro]

I'd prefer to make this change more future facing and robust to a time where pennylane has fully transition to graph based decompositions.

As long as we have a graph based decomposition registered, null qubit should be able to decompose it.

Are there any improvements we could make to has_decomp to make it more usable?

[JerryChen97]

I'd prefer to make this change more future facing and robust to a time where pennylane has fully transition to graph based decompositions.

As long as we have a graph based decomposition registered, null qubit should be able to decompose it.

Are there any improvements we could make to has_decomp to make it more usable?

Then I think we do need to lift the stopping condition of NQ?

[JerryChen97]

Or matbe Operations.has decompse needs update

[astralcai]

I'll approve but don't forget to update the code once my PR is merged.

[Alex-Preciado]

I'll approve but don't forget to update the code once my PR is merged.

@astralcai Please link the relevant PR

[JerryChen97]

I'll approve but don't forget to update the code once my PR is merged.

@astralcai Please link the relevant PR

I put it in the PR description