Add RUBYLLM_LOG_FILE env var

[Niraj22]

Allows setting the log destination from the environment, complementing RUBYLLM_DEBUG and RUBYLLM_STREAM_DEBUG. config.log_file now defaults to RUBYLLM_LOG_FILE when set, falling back to $stdout.

Includes specs and documentation updates.

Fixes #658

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.33%. Comparing base (b229895) to head (ddc7d6e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #836      +/-   ##
==========================================
- Coverage   83.34%   83.33%   -0.02%     
==========================================
  Files         172      172              
  Lines        8282     8282              
  Branches     1347     1347              
==========================================
- Hits         6903     6902       -1     
- Misses        886      887       +1     
  Partials      493      493              

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