Add OpenAPI 3 documentation through the existing REST pipeline

[vicancy]

Add OpenAPI 3.0, 3.1 and 3.2 JSON/YAML documentation using Microsoft.OpenApi and Microsoft.OpenApi.YamlReader 3.10.2. Related to #11151; builds on the Swagger 2.0 compatibility coverage in #11152 and #11181.

Shared document input probes JSON/YAML protocol markers and YamlMime once for processor selection and loading. Existing YamlMime processors use the same input. REST dispatch selects the existing Swagger 2.0 parser/converter or the OpenAPI 3 adapter; protocol readers receive the detected version and input instead of reopening the root file. Both produce the existing REST view models with extensible metadata dictionaries. The specification version is recorded separately from the API's info.version. Swagger 2.0 metadata access, split-page inheritance, overwrite behavior and allOf rendering retain their existing contracts.

The shared template transform and partials display operations, parameters, request bodies, responses, schemas and examples. OpenAPI 3 description markup visits the documented fields while preserving literal JSON. Coverage includes server overrides, references and cycles within one document, typed constants, null defaults, boolean schemas, and OpenAPI 3.2 QUERY/custom methods, streaming itemSchema and dataValue/serializedValue examples. Generated 3.0 and 3.2 examples are included in the documentation navigation.

Cross-file and network $ref targets are deferred and produce UnsupportedExternalReference. Known lossy OpenAPI 3.0 compositions are rejected, and features without dedicated documentation UI produce warnings. The documentation describes these limits.

Suggested review order:

1. Shared document input, JSON/YAML detection and processor dispatch.
2. The OpenAPI 3 adapter, metadata description markup and overwrite handling.
3. Shared template extensions, generated examples and compatibility tests.

Validation:

• 165 reader/model tests and 43 HTML/splitting tests pass on each of .NET 8 and .NET 10, including the unchanged Swagger 2.0 compatibility baseline.
• Shared input tests pass on .NET 8 and .NET 10. Existing YAML processor and build integration suites pass on .NET 10; three existing remote xref tests remain skipped.
• All 17 JavaScript tests, template lint and template build pass.
• Documentation build succeeds with zero warnings/errors; both example pages and the updated feature-limit page are served by the local preview.