Make HTTPSCallableResult Sendable to fix Swift 6 bridge data-race error

[IT-Guy007]

Summary

Fixes #92.

Since 0.20.0 bumped swift-tools-version to 6.1, the package and its skipstone-generated *_Bridge.swift files compile in Swift 6 language mode, where strict-concurrency violations are hard errors. The generated SkipFirebaseFunctions bridge resumes its continuation with the call result:

SkipFirebaseFunctions_Bridge.swift:253:36: error: sending 'f_return' risks causing data races [#SendingRisksDataRace]
253 |                     f_continuation.resume(returning: f_return!)
    |                                    |- error: sending 'f_return' risks causing data races

resume(returning:) takes a sending parameter under Swift 6, but the re-exported FirebaseFunctions.HTTPSCallableResult is not Sendable, so passing it across the task-isolation boundary is rejected. This compiled fine under 0.19.0 (tools-version 5.9, where it was at most a warning).

Change

Add a retroactive @unchecked Sendable conformance for HTTPSCallableResult, mirroring the exact pattern 0.20.0 already applied to Firestore, CollectionReference and Messaging — Functions was simply missed.

extension HTTPSCallableResult: @retroactive @unchecked Sendable {}

Verification

swift build --target SkipFirebaseFunctions (Darwin) compiles clean; the generated functions bridge no longer raises #SendingRisksDataRace.

[IT-Guy007]

@marcprux please approve and create version 0.20.1

[marcprux]

@marcprux please approve and create version 0.20.1

I followed-up at #92 (comment)

[IT-Guy007]

@marcprux the issue has been fixed with the latest skip version, up to you if you still want to merge this pr. Functions does miss extension HTTPSCallableResult: @retroactive @unchecked Sendable {} compared to the others.