Follow-up bug fixes for _group_measurements

[astralcai]

Context:
PR #5525 led to two bugs:

• group_observables uses are_identity_pauli_words, which does not work for legacy Hamiltonian.
• get_num_shots_and_executions incorrectly counts number of executions for Sum

Description of the Change:

• Converts single-term Hamiltonian to SProd
• Use the grouping information of Sum to correctly count number of executions.

Related Shortcut Issues:
[sc-61807]
[sc-61812]
Fixes #5561

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.68%. Comparing base (f504341) to head (3174ef7).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5559      +/-   ##
==========================================
- Coverage   99.69%   99.68%   -0.01%     
==========================================
  Files         410      410              
  Lines       38235    37987     -248     
==========================================
- Hits        38118    37869     -249     
- Misses        117      118       +1     

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[lillian542]

Looks good to me!

[PietropaoloFrisoni]

Seems flawless to me!