Add color-coded log formatter with level-aware ANSI colors

[Carreau]

Extract log formatting into a dedicated _PyflybyFormatter class that applies color based on log level: blue for info, yellow for warnings, and red for errors/critical. Previously all interactive output used a single hardcoded yellow prefix regardless of severity.

The formatter implements separate formatInteractive and formatPlain methods to keep the color logic out of the handler's emit method.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.15%. Comparing base (beedbdb) to head (76f3349).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #477      +/-   ##
==========================================
+ Coverage   75.10%   75.15%   +0.04%     
==========================================
  Files          29       29              
  Lines        8816     8833      +17     
==========================================
+ Hits         6621     6638      +17     
  Misses       2195     2195              

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