feat(gateway): OAS validation structured errors

[lena-larionova]

Description

Adds a section on error handling in the OAS Validation plugin and logs the new params in 3.15.

You can test with the 3.15.0.0-rc.3 image and the following snippet:

curl -i -X POST http://localhost:8001/plugins/ \
    --header "Accept: application/json" \
    --header "Content-Type: application/json" \
    --data '
    {
      "name": "oas-validation",
      "config": {
        "api_spec": "openapi: 3.0.0\ninfo:\n  title: Sample API\n  version: 1.0.0\npaths:\n  /test:\n    get:\n      responses:\n        \"200\":\n          description: OK",
        "structured_errors": true,
        "max_structured_errors": 10,
        "verbose_response": true
      },
      "tags": []
    }
    '

Fixes #5414

Preview Links

https://deploy-preview-5553--kongdeveloper.netlify.app/plugins/oas-validation/#error-handling
https://deploy-preview-5553--kongdeveloper.netlify.app/plugins/oas-validation/examples/structured-errors/

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[Copilot]

Pull request overview

Adds documentation for the OAS Validation plugin’s new 3.15 structured error reporting options, including a new example config users can apply.

Changes:

• Adds an Error handling section describing structured_errors and max_structured_errors and how they affect validation error output.
• Adds a new plugin example demonstrating how to enable structured validation errors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
app/_kong_plugins/oas-validation/index.md	Documents structured error output and related parameters for OAS Validation in 3.15.
app/_kong_plugins/oas-validation/examples/structured-errors.yaml	Adds a new plugin example enabling structured error reporting.