Gates in the gate set are not considered solved in the graph

[astralcai]

Context:
A KeyError is raised when trying to access the decomposition rule of an operator that is in the gate set.

Description of the Change:
Now the graph correctly returns False when is_solved_for is called on an op in the gate set, and the correct error is raised when trying to access the decomposition rule of such an operator.

Benefits:

Possible Drawbacks:

Related GitHub Issues:
[sc-102050]

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.42%. Comparing base (5442984) to head (7cce6a7).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8526      +/-   ##
==========================================
- Coverage   99.42%   99.42%   -0.01%     
==========================================
  Files         584      584              
  Lines       61293    61293              
==========================================
- Hits        60940    60939       -1     
- Misses        353      354       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[comp-phys-marc]

Makes sense!

[andrijapau]

Thanks!