refactor(rust): Make polars-plan constants more consistent

[Voultapher]

• Consistently use Use get_literal_name in implementation
• Consistently use const
• Use PlSmallString::from_static effectively

[Voultapher]

Literally every single place calling the get_.*_name() functions was cloning the result so I made the functions return an owned PlSmallString to reduce visual noise in the callers.

[codecov]

Codecov Report

❌ Patch coverage is 85.29412% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.68%. Comparing base (ae8a737) to head (4192ba2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/plans/lit.rs	28.57%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25645      +/-   ##
==========================================
+ Coverage   79.66%   79.68%   +0.02%     
==========================================
  Files        1743     1743              
  Lines      240457   240450       -7     
  Branches     3038     3038              
==========================================
+ Hits       191560   191604      +44     
+ Misses      48115    48063      -52     
- Partials      782      783       +1     

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