Fix backward_step node removal to clean up incident edges

[josephdviviano]

Summary

• When backward-stepping a node removal in GraphBuilding, edges incident to the deleted node must be removed before re-indexing
• Previously the code asserted no such edges existed (assert torch.all(graph.edge_index != node_idx)), crashing if the graph had edges touching the removed node
• Now edges are properly filtered out and edge_attr is cleaned up alongside edge_index
• Cherry-picked from the fix-graph2 branch with an additional edge_attr fix

Test plan

• test_graph_env passes
• Full test suite passes (1511 tests)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.48%. Comparing base (f0605a8) to head (342dce6).

Files with missing lines	Patch %	Lines
src/gfn/gym/graph_building.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
- Coverage   72.48%   72.48%   -0.01%     
==========================================
  Files          55       55              
  Lines        8519     8522       +3     
  Branches     1090     1091       +1     
==========================================
+ Hits         6175     6177       +2     
  Misses       1957     1957              
- Partials      387      388       +1     

Files with missing lines	Coverage Δ
src/gfn/gym/graph_building.py	81.90% <75.00%> (-0.21%)	⬇️

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