fix: OpenAPI Schema Issue for /upload Endpoint

[serinuntius]

Description

While trying to generate Rust code using the OpenAPI specification with the command openapi-generator-cli generate -g rust -i docs/api.yml -o hoge, I encountered the following validation error:

Exception in thread "main" org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 1, Warning count: 1
Errors: 
        -attribute paths.'/upload'(post).parameters.[x-agent-did].schemas.required is not of type `array`

This PR aims to resolve this issue by correcting the schema definition for the x-agent-did parameter in the /upload endpoint.