refactor: Replace portgraph toposort in CommandIterator with node vector

[aborgna-q]

Replaces the deprecated hugr.region_portgraph call by using hugr.scheduling_graph and eagerly collecting the toposorted node list.

This is necessary for #1580 since region_portgraph has been removed in the update.

I'll follow up this PR with another deprecating CommandsIterator.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.44%. Comparing base (d5890d6) to head (6f9c572).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1608      +/-   ##
==========================================
- Coverage   85.46%   85.44%   -0.02%     
==========================================
  Files         184      184              
  Lines       28772    28768       -4     
  Branches    27571    27567       -4     
==========================================
- Hits        24591    24582       -9     
- Misses       3054     3055       +1     
- Partials     1127     1131       +4     

Flag	Coverage Δ
python	90.45% <ø> (ø)
qis-compiler	91.66% <ø> (ø)
rust	85.22% <100.00%> (-0.03%)	⬇️

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