Add sendable annotation to userInfo closure

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).

Author: bnbarham

Sources/LanguageServerProtocolJSONRPC/JSONRPCConnection.swift

@@ -367,7 +367,9 @@ public final class JSONRPCConnection: Connection {
     decoder.userInfo[.messageRegistryKey] = messageRegistry
 
     // Setup callback for response type.
-    decoder.userInfo[.responseTypeCallbackKey] = { (id: RequestID) -> ResponseType.Type? in
+    decoder.userInfo[.responseTypeCallbackKey] = { @Sendable (id: RequestID) -> ResponseType.Type? in
+      // `outstandingRequests` should never be mutated in this closure. Reading is fine as all of our other writes are
+      // guarded by `queue`, but `JSONDecoder` could (since this is sendable) invoke this concurrently.
       guard let outstanding = self.outstandingRequests[id] else {
         logger.error("Unknown request for \(id, privacy: .public)")
         return nil

Sources/SKTestSupport/CheckCoding.swift

@@ -95,7 +95,7 @@ package func checkDecoding<T: Codable & Equatable>(
 package func checkCoding<T: Codable>(
   _ value: T,
   json: String,
-  userInfo: [CodingUserInfoKey: Any] = [:],
+  userInfo: [CodingUserInfoKey: any Sendable] = [:],
   file: StaticString = #filePath,
   line: UInt = #line,
   body: (T) -> Void