Fix OpenAPI 3.0 example in external files crashing OpenAPIPage
The version upgrader ran after external documents were embedded under x-ext, where it can no longer classify schemas by their JSON path: a schema-level example from an external 3.0 file became an Example Object map instead of the JSON Schema examples array, crashing the schema UI with schema.examples is not iterable.
Each document is now upgraded before bundling embeds it. This also honors the external file's own declared OpenAPI version, so a 3.0 file referenced from a 3.1 document is upgraded too (previously it was skipped entirely).