fix: Recompute RowIndex schema after projection pd

[ritchie46]

fixes #15614

[codspeed-hq]

CodSpeed Performance Report

Merging #15625 will degrade performances by 11.54%

_{Comparing schema (ab942b6) with main (92902e6)}

Summary

❌ 1 regressions
✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	schema	Change
❌	test_sort_nan_1942	1 ms	1.2 ms	-11.54%

[codecov]

Codecov Report

Attention: Patch coverage is 86.72566% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 81.26%. Comparing base (c068e76) to head (f9baf6f).
Report is 7 commits behind head on main.

❗ Current head f9baf6f differs from pull request most recent head ab942b6. Consider uploading reports for the commit ab942b6 to get more accurate results

Files	Patch %	Lines
...polars-lazy/src/physical_plan/executors/hconcat.rs	0.00%	3 Missing ⚠️
...s/polars-plan/src/logical_plan/functions/schema.rs	96.42%	3 Missing ⚠️
...s/polars-lazy/src/physical_plan/executors/union.rs	50.00%	2 Missing ⚠️
...rs-lazy/src/physical_plan/executors/ext_context.rs	0.00%	1 Missing ⚠️
...olars-lazy/src/physical_plan/executors/group_by.rs	0.00%	1 Missing ⚠️
...zy/src/physical_plan/executors/group_by_dynamic.rs	0.00%	1 Missing ⚠️
...zy/src/physical_plan/executors/group_by_rolling.rs	0.00%	1 Missing ⚠️
...rs-lazy/src/physical_plan/executors/python_scan.rs	0.00%	1 Missing ⚠️
crates/polars-plan/src/logical_plan/builder.rs	0.00%	1 Missing ⚠️
crates/polars-plan/src/logical_plan/builder_alp.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15625      +/-   ##
==========================================
+ Coverage   81.20%   81.26%   +0.05%     
==========================================
  Files        1371     1372       +1     
  Lines      175822   175821       -1     
  Branches     2531     2543      +12     
==========================================
+ Hits       142782   142873      +91     
+ Misses      32563    32466      -97     
- Partials      477      482       +5     

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