refactor: Update Cloud plan preparation

[stinodego]

This introduces a (private) utility function that takes a LazyFrame, adds a sink, checks that it's eligible to be run on Polars Cloud, then serializes it and returns the bytes.

[ritchie46]

Nice, a few small comments.

[codecov]

Codecov Report

Attention: Patch coverage is 88.57143% with 8 lines in your changes missing coverage. Please review.

Project coverage is 80.45%. Comparing base (66f0026) to head (1a9bdd6).
Report is 4 commits behind head on main.

Files	Patch %	Lines
crates/polars-plan/src/client/check.rs	81.08%	7 Missing ⚠️
crates/polars-plan/src/client/mod.rs	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17802      +/-   ##
==========================================
- Coverage   80.47%   80.45%   -0.03%     
==========================================
  Files        1503     1503              
  Lines      197115   196977     -138     
  Branches     2794     2794              
==========================================
- Hits       158628   158474     -154     
- Misses      37973    37989      +16     
  Partials      514      514              

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