Add head property to FileDownloadDelegate's Progress/Response struct

[gregcotten]

I needed a way to use a FileDownloadDelegate task to fish out the recommended file name.

let response = try await downloadTask.get()

// access content-disposition
response.head.headers.first(name: "Content-Disposition")

The head property is an explicitly unwrapped optional because there is no "default value" to set it to, and it won't be accessed by the user until it's already been set anyway. This is a little inelegant, so I could change it to something like below where I fill in bogus init data, but that seems worse for some reason.

public struct Progress: Sendable {
    public var totalBytes: Int?
    public var receivedBytes: Int
    public var head: HTTPResponseHead
}

private var progress = Progress(
    totalBytes: nil,
    receivedBytes: 0,
    head: .init(
        version: .init(major: 0, minor: 0),
        status: .badRequest
    )
)

[gregcotten]

summoning @Lukasa and @dnadoba since this was discussed previously here: #680 (comment)

[Lukasa]

Hi @gregcotten, thanks for opening this and sorry for the slow time to review, it's been a busy week. I'll take a look now.

[Lukasa]

Great, left a note in the diff. Can we also add at least one unit test?

[gregcotten]

OK @Lukasa I, at my own risk, have diverged from your suggestion as I think it's really important that the head property is never nil (since from the user's perspective it should never be). I've instead initialized it with bogus data (HTTP 0.0, status code 0) and it's replaced before the library user ever gets a hold of it.

I've also added its use to existing unit tests, but can make a unique unit test if you have any suggestions.

[gregcotten]

Needless to say this should be a squash merge if accepted :)

[Lukasa]

Very minor nits here but this is looking good. I'll kick off CI for some more info there as well.

[Lukasa]

Looks like the formatter wants a few tweaks from you.

[gregcotten]

Looks like the formatter wants a few tweaks from you.

Done!

[Lukasa]

Awesome, I really like this.