refactor: Remove extra hashmap construction in new-streaming parquet

[nameexhaustion]

ArrowSchema lookup is now O(1)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 79.84%. Comparing base (f631502) to head (c895a2e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...-stream/src/nodes/parquet_source/metadata_utils.rs	0.00%	6 Missing ⚠️
...-stream/src/nodes/parquet_source/metadata_fetch.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18792   +/-   ##
=======================================
  Coverage   79.83%   79.84%           
=======================================
  Files        1518     1518           
  Lines      205567   205563    -4     
  Branches     2892     2892           
=======================================
+ Hits       164124   164135   +11     
+ Misses      40895    40880   -15     
  Partials      548      548           

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