feat: Add positive slice for new streaming MultiScan

[coastalwhite]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 90.77491% with 25 lines in your changes missing coverage. Please review.

Project coverage is 79.86%. Comparing base (9bcff72) to head (7442f06).
Report is 34 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/polars-stream/src/nodes/io_sources/multi_scan.rs	93.36%	15 Missing ⚠️
crates/polars-stream/src/nodes/io_sources/csv.rs	76.19%	5 Missing ⚠️
crates/polars-mem-engine/src/predicate.rs	0.00%	3 Missing ⚠️
crates/polars-stream/src/nodes/io_sources/mod.rs	90.90%	1 Missing ⚠️
...polars-stream/src/nodes/io_sources/parquet/init.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21191      +/-   ##
==========================================
+ Coverage   79.75%   79.86%   +0.10%     
==========================================
  Files        1593     1593              
  Lines      228119   228268     +149     
  Branches     2600     2600              
==========================================
+ Hits       181947   182298     +351     
+ Misses      45575    45373     -202     
  Partials      597      597              

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