Add sendable annotation to userInfo closure

[bnbarham]

swift-foundation recently landed a change (in
swiftlang/swift-foundation#764) which requires any Sendable values in JSONEncoder.userInfo. This causes a build failure:

JSONRPCConnection.swift:370:50: error: type '(RequestID) -> Optional<any ResponseType.Type>' does not conform to the 'Sendable' protocol
368 |
369 |     // Setup callback for response type.
370 |     decoder.userInfo[.responseTypeCallbackKey] = { (id: RequestID) -> ResponseType.Type? in
    |                                                  |- error: type '(RequestID) -> Optional<any ResponseType.Type>' does not conform to the 'Sendable' protocol
    |                                                  `- note: a function type must be marked '@Sendable' to conform to 'Sendable'
371 |       guard let outstanding = self.outstandingRequests[id] else {
372 |         logger.error("Unknown request for \(id, privacy: .public)")

Make the closure sendable, which is safe as we're only reading from outstandingRequests (where all our writes are guarded by the same queue that the decoding is on).

[bnbarham]

@swift-ci please test Windows platform

[bnbarham]

(note: completely untested, just running Windows since Linux will fail with docc first)

[ahoppen]

Marking the closure as Sendable implies that JSONDecoder should be able to invoke it multiple times concurrently when deserializing the JSON at which point we would get concurrent accesses to outstandingRequests. I think that should be fine because they are all just reading, which should be fine (StackOverflow seems to imply so https://stackoverflow.com/questions/45666181/is-it-safe-to-access-not-modify-a-swift-dictionary-from-multiple-threads) because the lock on queue guarantees that there are no concurrent writes.

I think it would be good to include that rationale in a comment so that we don't accidentally start modifying outstandingRequests in that closure in the future.

[bnbarham]

swiftlang/swift#79380

@swift-ci please test

[ahoppen]

swiftlang/swift#79380

@swift-ci please test Windows

[bnbarham]

@swift-ci please test Windows platform

[bnbarham]

@swift-ci please test

[bnbarham]

@swift-ci please test Windows platform

[bnbarham]

@swift-ci please test Windows platform

[bnbarham]

@swift-ci please test macOS platform