[API Explorer] Add support for x-req-auth

[lcawl]

Summary

Adds support for the OpenAPI operation extension x-req-auth in the API Explorer.
Render the information in a Prerequisites block on each operation page.

NOTE: x-req-auth is implemented in the Elasticsearch specification in elastic/elasticsearch-specification#6007

Screenshots

Implementation

• OpenApiXReqAuthParser: Reads x-req-auth as a JSON array of strings on the operation. Trims each line; non-string JSON values are stringified. Returns null if the key is missing, not an array, or all entries are empty—so the UI stays hidden. Logs warnings (when a logger is available) for unexpected shapes or parse issues.
• ApiRenderContext: New optional ApiExplorerLog (ILogger?) for extension parsing.
• OpenApiGenerator: Wires ApiExplorerLog = _logger when constructing ApiRenderContext for API product generation.
• ApiViewModel: Exposes protected ApiRenderContext RenderContext so operation code can use the model and optional logger.
• OperationViewModel:
  • RequiredAuthItems from the parser.
  • GetTocItems: Inserts Prerequisites (anchor prerequisites) after Paths when there is at least one line; coexists with existing Code Examples / x-codeSamples TOC behavior.
• OperationView.cshtml: Renders the Prerequisites section (same general styling as other primary operation sections).

Tests

• XReqAuthTests: Parser coverage (minimal spec, empty array, Elasticsearch- and Kibana-style TestData JSON fixtures—no dependency on the full repo elasticsearch-openapi.json / kibana-openapi.json).
• Test project copies TestData/**/*.json to output.

Documentation

• docs/configure/content-set/api-explorer.md: Documents x-req-auth under OpenAPI extensions (format, behavior, TOC placement, when the section is omitted, warnings), and lists it in the extensions overview.

I've done some minor cleanup in that page as well (re-ordering the sections under "OpenAPI extensions".

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: composer-2