fix: Fix errors on native scan_iceberg

[nameexhaustion]

Changes

• Fixes Panic when collecting the same native Iceberg scan twice #23841
  • The IR was not overwritten on the 2nd execution of the query
• Fixes Native Iceberg scan fails on test_scan_iceberg_plain #23842
  • This file did not have ARROW:schema, causing us to go into a separate codepath for constructing the schema. This codepath has been updated in this PR to load the field IDs.

[nameexhaustion]

We always want to mutate here, but we don't want the StackOptimizer to re-call us.

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.41%. Comparing base (4b0f8dd) to head (27cdb8d).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...es/polars-parquet/src/arrow/read/schema/convert.rs	46.15%	7 Missing ⚠️
...polars-plan/src/plans/optimizer/expand_datasets.rs	88.63%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23811      +/-   ##
==========================================
+ Coverage   81.38%   81.41%   +0.02%     
==========================================
  Files        1653     1653              
  Lines      224519   224506      -13     
  Branches     2873     2873              
==========================================
+ Hits       182718   182773      +55     
+ Misses      41087    41019      -68     
  Partials      714      714              

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