Raise an error for MCMs nested in Conditionals

[lillian542]

Context:
The current methods for of mid-circuit measurement processing don't support applying a mid-circuit measurement conditionally. This was not within the scope of the implementation for any of these methods, so we don't expect it to work - however, we would prefer to return clear error messages, and we don't want to be returning incorrect results.

Currently, qml.cond(m, qml.measure)(wire) either raises a difficult to parse error (in the case of deferred measurements and analytical execution), or returns incorrect results (in the case of tree-traversal, one-shot, and deferred measurements with shots).

Description of the Change:

We add validation in the midcircuit_measurements transform, checking for conditional mcms and raising an error. Specific mcm_methods can be made to pass this validation check in the future if support for conditional mcms are added to the method; for now, we disallow it completely.

Benefits:
We don't return incorrect results or raise cryptic errors.

Possible Drawbacks:
It adds one more iteration over the operations for validation 🤷‍♀️

Related GitHub Issues:
None, but sc stories [sc-84177] and [sc-84178]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.60%. Comparing base (9b45754) to head (d67b42b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7027   +/-   ##
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  Coverage   99.60%   99.60%           
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  Files         484      484           
  Lines       46305    46312    +7     
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+ Hits        46122    46129    +7     
  Misses        183      183           

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