Error when implementing a middleware with SWIFT_APPROACHABLE_CONCURRENCY

### Question

Since Xcode 26 beta 5 i cannot comply with the `ClientMiddleware` protocol of `swift-openapi-runtime`. The error that I got is `Candidate has non-matching type '(HTTPRequest, HTTPBody?, URL, String, nonisolated(nonsending) @Sendable (HTTPRequest, HTTPBody?, URL) async throws -> (HTTPResponse, HTTPBody?)) async throws -> (HTTPResponse, HTTPBody?)'` for a structure that seems (and was) to be well "made":

```swift
public struct ApiKeyMiddleware: ClientMiddleware {

//The API key to login
private let apiKey: String


public init(apiKey: String) { self.apiKey = apiKey }

public func intercept(
_ request: HTTPRequest,
body: HTTPBody?,
baseURL: URL,
operationID: String,
next: @Sendable (HTTPRequest, HTTPBody?, URL) async throws -> (
HTTPTypes.HTTPResponse, OpenAPIRuntime.HTTPBody?
)
) async throws -> (HTTPTypes.HTTPResponse, OpenAPIRuntime.HTTPBody?) {
var request = request
request.headerFields[.init("x-goog-api-key")!] = self.apiKey
return try await next(request, body, baseURL)
}
}
```

Is this an Xcode/Swift bug or is there something that I'm missing?

Thanks

Start at the ClientMiddleware protocol and compare its intercept signature with the provided ApiKeyMiddleware implementation under Xcode 26 beta 5 and SWIFT_APPROACHABLE_CONCURRENCY. Reproduce the candidate mismatch using the example; done means determining whether the protocol, implementation, or Xcode/Swift behavior is responsible.

Written by the indexing model from the issue text.