Conform AnyAsyncSequence to Sendable

[yamada-k-25]

Description

I defined an AsyncPassthroughSubject as a private property and exposed it as an AnyAsyncSequence by performing type erasure with .eraseToAnyAsyncSequence(). For example:

actor SampleActor {
    private let numbersSubject = AsyncPassthroughSubject<Int>()
    public var numbers: AnyAsyncSequence<Int> {
        self.numbersSubject.eraseToAnyAsyncSequence()
    }
    ...
}

I then use this actor to retrieve values from numbers like so:

let sampleActor = SampleActor()

for try await number in await sampleActor.numbers {
    ...
}

However, in my project I am using Swift 6, where the compiler enforces Sendable checking. Since AnyAsyncSequence does not conform to Sendable, the following error occurs:

Non-sendable type 'AnyAsyncSequence<Int>' in implicitly asynchronous access to actor-isolated property 'numbers' cannot cross actor boundary

Although it's possible to work around this issue on the user side (for example, by adding extension AnyAsyncSequence: @unchecked Sendable {} or by marking the property as public nonisolated var numbers: AnyAsyncSequence<Int>), I would like to resolve this problem within the library itself.

Checklist

• this PR is based on the main branch and is up-to-date, if not please rebase your branch on the top of main
• the commits inside this PR have explicit commit messages
• unit tests cover the new feature or the bug fix
• the feature is documented in the README.md if it makes sense
• the CHANGELOG is up-to-date