Xcode 26 Beta 3 - Main actor-isolated conformance cannot satisfy conformance requirement for a 'Sendable' type parameter 'Self'

### Description

While following the [Generating a client in an Xcode project](https://swiftpackageindex.com/apple/swift-openapi-generator/1.9.0/tutorials/swift-openapi-generator/clientxcode#Configuring-your-target-to-use-the-Swift-OpenAPI-Generator-plugin) tutorial, I ran into issues at the build step

`Main actor-isolated conformance of 'Operations.GetGreeting.AcceptableContentType' to 'CaseIterable' cannot satisfy conformance requirement for a 'Sendable' type parameter 'Self'
`

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After digging around, It seems that the default Swift Compiler build settings for Concurrency have changed in Xcode 26 when creating a new project

**Swift Compiler - Concurrency**

| Setting | < Xcode 26 | Xcode 26 |
| --- | --- | --- |
| Approachable Concurrency | No | Yes |
| Default Actor Isolation | nonisolated | MainActor |

✅ _**Switching `Default Actor Isolation` back to `nonisolated` enables the project to build successfully**_ ✅

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### Reproduction

Follow the [Generating a client in an Xcode project](https://swiftpackageindex.com/apple/swift-openapi-generator/1.9.0/tutorials/swift-openapi-generator/clientxcode#Configuring-your-target-to-use-the-Swift-OpenAPI-Generator-plugin) tutorial using Xcode 26, creating a new project

### Package version(s)

swift-openapi-generator: 1.10.0

### Expected behavior

To build and use a client within an iOS App

### Environment

Xcode: Version 26.0 beta 3 (17A5276g)
swift-driver version: 1.127.8 Apple Swift version 6.2 (swiftlang-6.2.0.13.10 clang-1700.3.13.4)
Target: arm64-apple-macosx26.0

### Additional information

Found the same [issue](https://github.com/apple/swift-openapi-generator/issues/781#issuecomment-2993961358) reported by @mikaelacaron in a separate thread

cc: @czechboy0

Reproduce the failure by following the “Generating a client in an Xcode project” tutorial with swift-openapi-generator 1.10.0 and Xcode 26 beta 3. Start from the generated Operations.GetGreeting.AcceptableContentType conformance and compare behavior with Xcode’s Approachable Concurrency and Default Actor Isolation settings. Done means the generated client builds in a new Xcode 26 project using the default settings.

Written by the indexing model from the issue text.