Change casting rules for StateMP to avoid copy in Lightning

[vincentmr]

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---

Context:
The changes made in #5547 led to a performance regression in Lightning because a superfluous state vector copy is made when adding 0.j, which was introduced to cast the output to complex.

Description of the Change:
Use qml.math.cast instead.

Benefits:
Avoid unnecessary sum of 0.j

Possible Drawbacks:

Related GitHub Issues:
[sc-65127]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (94b5533) to head (b20cb0e).
Report is 290 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5995      +/-   ##
==========================================
- Coverage   99.66%   99.65%   -0.01%     
==========================================
  Files         427      427              
  Lines       40983    40694     -289     
==========================================
- Hits        40845    40555     -290     
- Misses        138      139       +1     

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[AmintorDusko]

Looks good!