Refactor DecompositionGraph to two classes

[astralcai]

Context:

Description of the Change:
Splitting the solution part of a DecompositionGraph to a separate DecompGraphSolution class, returned by the solve method.

Benefits:
Cleaner code

Possible Drawbacks:

Related GitHub Issues:
[sc-97093]

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.68%. Comparing base (56c74cb) to head (3320996).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8031      +/-   ##
==========================================
- Coverage   99.68%   99.68%   -0.01%     
==========================================
  Files         543      543              
  Lines       56204    56220      +16     
==========================================
+ Hits        56029    56044      +15     
- Misses        175      176       +1     

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[maliasadi]

Thanks @astralcai! Just a few suggestions before approving the PR

[maliasadi]

LGTM! 🙌

[albi3ro]

Changelog?

But thanks for making this split! Great how well it worked right out of the box 🚀