refactor: Move type coercion to IR conversion phase

[ritchie46]

Now the schema is properly resolved before we start optimizing. Also has some drive-by cleanups.

[codecov]

Codecov Report

Attention: Patch coverage is 99.15966% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.34%. Comparing base (3f58c8f) to head (31c1abb).
Report is 7 commits behind head on main.

Files	Patch %	Lines
...src/logical_plan/conversion/dsl_plan_to_ir_plan.rs	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15868      +/-   ##
==========================================
- Coverage   81.37%   81.34%   -0.03%     
==========================================
  Files        1378     1385       +7     
  Lines      176827   176977     +150     
  Branches     3032     3033       +1     
==========================================
+ Hits       143889   143966      +77     
- Misses      32451    32526      +75     
+ Partials      487      485       -2     

Flag	Coverage Δ
python	74.71% <94.87%> (-0.03%)	⬇️
rust	78.47% <99.15%> (-0.03%)	⬇️

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