Support Opaque annotations in schema generation (#3001)
* feat: export Opaque annotation marker
* fix: remove unused Opaque test import
* feat: support Opaque in runtime field types
* fix: preserve Opaque bare list field shape
* feat: preserve Opaque metadata in inspector
* fix: preserve non-opaque Annotated inspector behavior
* fix: recurse Annotated stripping in inspector
* feat: support Opaque in static input schema
* fix: preserve Opaque optional list shapes
* fix: preserve qualified Opaque input shapes
* fix: restrict Opaque shape recognition to typing
* feat: support Opaque in static output schema
* fix: handle Opaque output list shapes
* docs: suggest Opaque for unresolved external types
* docs: document Opaque annotations
* docs: clarify Opaque serialization behavior
* fix: address Opaque verification lint failures
* fix: align Opaque output annotation handling
* fix: allow optional Annotated model fields
* fix: preserve opaque object field schemas
* fix: preserve optional object field schemas
* fix: omit empty output schema required
* fix: address opaque schema review feedback
* fix: simplify opaque import detection
