fix: operation callbacks shape

[danielgtaylor]

This fixes a mistake with how the Operation.Callbacks get defined. It should be a map of name to map of URL expression to path item object, as described in the linked issue. This is only used by external tools, so no tests within Huma for now.

Fixes #653.

Copilot reviewed 1 out of 1 changed files in this pull request and generated no suggestions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.76%. Comparing base (4f98ee6) to head (375568c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #665   +/-   ##
=======================================
  Coverage   92.76%   92.76%           
=======================================
  Files          22       22           
  Lines        4905     4905           
=======================================
  Hits         4550     4550           
  Misses        308      308           
  Partials       47       47           

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