Fix dd issue in calling PauliString.after.

[babacry]

Previous dd function would crash for circuits with PhXZ(a=0.25,x=-1,z=0) gate, e.g.,

   a: ───H───────PhXZ(a=0.25,x=-1,z=0)───────@───
                                             │
   b: ───H───H───H───────────────────────H───X───

in calling PauliString.after(ops) function. It is due to a known issue of decomposition.

In the PR:

1. Force ops passed to PauliString in the format of CliffordGate.on(ops).
2. Test cases are added to make sure the fix works.

Tracking issue: #6939

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.86%. Comparing base (988ecbc) to head (7deb8a8).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6943   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files        1084     1084           
  Lines       94408    94420   +12     
=======================================
+ Hits        92393    92406   +13     
+ Misses       2015     2014    -1     

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