Make PathItem.Build() a bit more robust to malformed input

[justinclift-prvidr]

Stumbled over a panic when running vacuum on a slightly invalid openapi.yml file, which had an empty parameters: line under an operation, with no nodes underneath.

Tracing that back, it turns out libopenapi does a two pass construction in PathItem.Build() when creating the model and when it hits a problem it can leave nil references in the model.

The consumer of this (doctor → vacuum in this case) may not handle a model with nil references well (ie panic).

This PR takes the fairly simple approach of making this code more robust so the construction continues to completion, returning an errors.Join() of all the failing operations and avoiding leaving nil references. This should help consumers avoid these panics.

Weirdly (?), Paths.Build() seems to swallow errors so this problem can't be easily detected by consumers of the model. That seems to be a deliberate design choice (with a test for it!), so this PR just works with it. 😄

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.75%. Comparing base (0837c9b) to head (793fa3e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
- Coverage   99.78%   99.75%   -0.03%     
==========================================
  Files         283      283              
  Lines       34456    34460       +4     
==========================================
- Hits        34382    34376       -6     
- Misses         46       55       +9     
- Partials       28       29       +1     

Flag	Coverage Δ
unittests	99.75% <100.00%> (-0.03%)	⬇️

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[justinclift-prvidr]

Hmmm, that looks like a false coverage failure?