chore: Dispatch to the in-memory engine for multifile sources

[coastalwhite]

This is very suboptimal, but necessary for now.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.64%. Comparing base (24fccc4) to head (a634327).
Report is 45 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-stream/src/physical_plan/lower_ir.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20860   +/-   ##
=======================================
  Coverage   79.64%   79.64%           
=======================================
  Files        1571     1571           
  Lines      223156   223169   +13     
  Branches     2546     2546           
=======================================
+ Hits       177731   177744   +13     
  Misses      44841    44841           
  Partials      584      584           

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