refactor(rust): Only instantiate used portion of graph

[orlp]

During our lowering to a PhysicalPlan we end up producing some physical plan nodes that end up going unused (e.g. because a node ends up being input independent). We ended up spawning execution graph nodes for this which caused failed asserts, etc.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.42%. Comparing base (c37bad2) to head (3f1dc7f).
Report is 14 commits behind head on main.

Files	Patch %	Lines
crates/polars-stream/src/physical_plan/to_graph.rs	0.00%	2 Missing ⚠️
crates/polars-stream/src/skeleton.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18337      +/-   ##
==========================================
- Coverage   80.42%   80.42%   -0.01%     
==========================================
  Files        1492     1492              
  Lines      198644   198676      +32     
  Branches     2837     2841       +4     
==========================================
+ Hits       159769   159792      +23     
- Misses      38349    38359      +10     
+ Partials      526      525       -1     

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