Remove redundant @unchecked Sendable annotations

[ianegordon]

Summary

Removes @unchecked Sendable annotations from four sites where the declaration is either redundant (the containing type is already Sendable by other means) or correct with a plain Sendable conformance (all contained values are Sendable).

File:Line	Before	After
HTTPResponse.swift:40	public enum Payload: @unchecked Sendable	public enum Payload: Sendable
WSMessage.swift:34	public enum WSMessage: @unchecked Sendable, Hashable	public enum WSMessage: Sendable, Hashable
HTTPConnection.swift:113	actor HTTPRequestSequence<...>: AsyncSequence, AsyncIteratorProtocol, @unchecked Sendable	actor HTTPRequestSequence<...>: AsyncSequence, AsyncIteratorProtocol
Logging+OSLog.swift:36	public struct OSLogLogger: Logging, @unchecked Sendable	public struct OSLogLogger: Logging

Rationale (primary-source verified)

• HTTPResponse.Payload: .httpBody(HTTPBodySequence) — HTTPBodySequence: Sendable (HTTPBodySequence.swift:35). .webSocket(any WSHandler) — WSHandler: Sendable (WSHandler.swift:34).
• WSMessage: associated values are String, Data, WSCloseCode. WSCloseCode: Sendable (WSCloseCode.swift:34); String / Data are Sendable.
• HTTPRequestSequence: per SE-0306, "every actor type implicitly conforms to the Sendable protocol." Explicit @unchecked Sendable (or even explicit Sendable) on an actor is redundant.
• OSLogLogger: os.Logger is declared public struct Logger : @unchecked Swift.Sendable in the Xcode SDK os.swiftmodule interface, and the Logging protocol already requires Sendable (Logging.swift:32) — so both the @unchecked and the explicit Sendable conformance were redundant.

Test plan

• swift build clean (no warnings, 5.16 s).
• swift test — 404 tests across 49 suites, all pass (1.13 s total).
• CI on the upstream repo.

Scope

This is Group A of a broader @unchecked Sendable audit. The remaining 12 sites fall into two categories:

• Group B — replace with synchronized state (e.g., Mutex-wrapped storage): HTTPBodySequence.Storage, AsyncStream+WSFrame.Iterator, WSFrameValidator.Validator. To be submitted separately.
• Group C — keep @unchecked with a justifying comment (Transferring, Mutex variants, ConsumingAsyncSequence.SharedBuffer/State, retroactive sockaddr_* conformances). To be submitted separately.

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