Check HTTP response status codes in fetch()

[bamnet-bot]

Fixes #12, #18

[gemini-code-assist]

Code Review

This pull request introduces structured error handling for non-2xx HTTP responses in the NJTransit API client by defining an APIError struct and ErrUnexpectedStatus error, along with corresponding unit tests. The reviewer suggests falling back to http.StatusText when the error response body is empty to improve error message readability, and truncating the stored error body to prevent high memory usage and log pollution from large upstream error responses.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.01%. Comparing base (250c6e8) to head (4d30d57).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   84.24%   85.01%   +0.76%     
==========================================
  Files           5        5              
  Lines         273      287      +14     
==========================================
+ Hits          230      244      +14     
  Misses         26       26              
  Partials       17       17              

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