End-to-end client validation tests

[martincostello]

Add a working Minimal API (the venerable "Todo" app) that is used to:

• generate an OpenAPI document
• generate a Kiota client from that OpenAPI document
• verify functional interactions with the API using that generated client code

This was then used to create a repro for #3683, which is then fixed.

This also identified, via the use of https://apinotes.io/openapi-validator, that nullable shouldn't be emitted for OpenAPI 3.0 if type isn't, so that has also been resolved. Unfortunately I haven't yet found a validator that flags that that can be used in CI.

Also:

• extends the tests for client generation with Kiota and NSwag to add code snapshots
• adds a lint step using Spectral to validate any OpenAPI schema snapshots have no errors
• validates the OpenAPI schema snapshots against their relevant OAS JSON schemas

TODO

• Investigate adding snapshot tests for the NSwag and Kiota clients added by Validate OpenAPI documents create valid C# clients #3686
• Investigate using IBM/openapi-validator or spectral to validate the OpenAPI snapshots

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.66%. Comparing base (ed88a1b) to head (09c7c95).
⚠️ Report is 15 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3687      +/-   ##
==========================================
+ Coverage   94.64%   94.66%   +0.02%     
==========================================
  Files         111      111              
  Lines        3865     3861       -4     
  Branches      780      779       -1     
==========================================
- Hits         3658     3655       -3     
+ Misses        207      206       -1     

Flag	Coverage Δ
Linux	94.66% <100.00%> (+0.02%)	⬆️
Windows	94.66% <100.00%> (+0.02%)	⬆️
macOS	94.66% <100.00%> (+0.02%)	⬆️

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[Copilot]

Pull request overview

This PR adds comprehensive end-to-end testing for client code generation from OpenAPI documents. A Todo API application serves as the test subject, with generated clients from both Kiota and NSwag validated for correctness. The PR also fixes issue #3683 related to nullable property emission and adds OpenAPI schema validation using Spectral.

• Adds a working Todo Minimal API used to generate and test OpenAPI clients
• Introduces code snapshots for generated Kiota and NSwag clients
• Implements OpenAPI schema validation with Spectral and JSON schema validation

Reviewed changes

Copilot reviewed 160 out of 315 changed files in this pull request and generated no comments.

File	Description
Multiple snapshot files	Generated client code snapshots for NSwag and Kiota clients across various test scenarios
.spectral.yaml	Configuration file enabling Spectral-based OpenAPI validation

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