Generalize DensePauliString.mul to handle multi-qubit Paulis

[daxfohl]

Instead of special blocks to handle paulis on linequbits in each of mul, rmul, imul, and commutes, this PR just converts any paulis to DensePauliString up-front. That allows removal of the special handling blocks, plus generalizes to multi-qubit paulis and eliminates out-of-bounds errors.

Fixes #7640

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.37%. Comparing base (25d727f) to head (c080723).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7643      +/-   ##
==========================================
- Coverage   99.37%   99.37%   -0.01%     
==========================================
  Files        1078     1078              
  Lines       96166    96162       -4     
==========================================
- Hits        95563    95558       -5     
- Misses        603      604       +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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[pavoljuhas]

LGTM with a couple of minor suggestions.

Thank you for taking care of this!