convert claude (anthropic) mutator to use the messages API

[batmac]

to be able to use claude 3 models
(see https://docs.anthropic.com/claude/reference/messages_post)

[codecov]

Codecov Report

Attention: Patch coverage is 26.89076% with 87 lines in your changes are missing coverage. Please review.

Project coverage is 57.67%. Comparing base (353f2d2) to head (956817d).

Files	Patch %	Lines
pkg/miniclaude/messagesapi.go	8.88%	82 Missing ⚠️
pkg/mutators/single/claude.go	82.75%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #613      +/-   ##
==========================================
+ Coverage   53.99%   57.67%   +3.67%     
==========================================
  Files         110      103       -7     
  Lines        4167     3896     -271     
==========================================
- Hits         2250     2247       -3     
+ Misses       1665     1395     -270     
- Partials      252      254       +2     

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