implement native multi-error aggregation (#34)

[cinar]

This PR implements native multi-error aggregation using Go 1.20's errors.Join in both standard and hedged retry loops. This prevents the loss of critical debugging context by returning the complete timeline of failures encountered during an exhausted retry sequence. fixes #34

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.47%. Comparing base (5d367eb) to head (b02f308).

Files with missing lines	Patch %	Lines
retry.go	87.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
+ Coverage   83.19%   83.47%   +0.27%     
==========================================
  Files          12       12              
  Lines         851      853       +2     
==========================================
+ Hits          708      712       +4     
+ Misses        115      114       -1     
+ Partials       28       27       -1     

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