refactor: Streamline internal SQL join condition processing

[alexander-beedie]

Improved SQL "JOIN ON" constraint resolution; there was a lot of redundant code in process_join_constraint - now delegates properly to process_join_on, noticeably streamlining it (and making it easier to integrate future join updates).

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.86%. Comparing base (6b0a906) to head (b217680).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-sql/src/context.rs	92.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19658   +/-   ##
=======================================
  Coverage   79.86%   79.86%           
=======================================
  Files        1537     1537           
  Lines      211923   211899   -24     
  Branches     2446     2446           
=======================================
- Hits       169249   169240    -9     
+ Misses      42120    42105   -15     
  Partials      554      554           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.