Add Sendable Conformance to Field and Reference Types (#353)

* Add Sendable conformance to Field property wrapper

Makes the Field<C> property wrapper Sendable-compliant to better support concurrent operations and Swift's actor isolation system.

* fix: make Field property wrapper conditionally Sendable

- Remove direct Sendable conformance from Field struct
- Add conditional Sendable conformance via extension where C: Sendable

* fix: add Sendable conformance to Reference struct

Sources/Meow/KeyPathModel/KeyPathModels.swift

@@ -361,6 +361,9 @@ public struct Field<C: Codable>: Codable, _QueryableFieldRepresentable {
     }
 }
 
+// Add conditional Sendable conformance
+extension Field: Sendable where C: Sendable {}
+
 extension Field: Equatable where C: Equatable {
     public static func ==(lhs: Self, rhs: Self) -> Bool {
         lhs.wrappedValue == rhs.wrappedValue

Sources/Meow/Reference.swift

@@ -13,7 +13,7 @@ import NIO
 ///         let post: Post = try await postRef.resolve(in: req.meow)
 ///         return post
 ///     }
-public struct Reference<M: ReadableModel>: Resolvable, Hashable, PrimitiveEncodable {
+public struct Reference<M: ReadableModel>: Resolvable, Hashable, PrimitiveEncodable, Sendable where M.Identifier: Sendable {
     /// The referenced id
     public let reference: M.Identifier