Raise error when applying Snapshot with measurements on default.mixed

[astralcai]

Context:
default.mixed device does not support Snapshot with measurements. It returns the density matrix, which is incorrect.

Description of the Change:
Adds an error message to when Snapshot with measurements is used on default.mixed.

Benefits:
Less confusion.

Possible Drawbacks:
New error raised. This is a near-term fix until we improve default.mixed to properly support Snapshot

Related GitHub Issues:
#5202
[sc-56793]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.63%. Comparing base (186faaf) to head (733908d).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5416      +/-   ##
==========================================
- Coverage   99.63%   99.63%   -0.01%     
==========================================
  Files         399      399              
  Lines       37112    36830     -282     
==========================================
- Hits        36977    36694     -283     
- Misses        135      136       +1     

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[trbromley]

Thanks @astralcai!