[api-upload-refactor] Add test target, relax decode constraints, add HTTPBody + MultipartForm

- Package: new LCEssentialsTests target (runs on iOS simulator)
- JSONDecoder.decode: T: Codable -> T: Decodable & Sendable (4 overloads);
  remove try! in decode(fromURL:); honour encoding param in decode(_:using:)
- Propagate & Sendable to Data.object, Dictionary.toObjetct, API.request return
- LCEHTTPBody: HTTPBody protocol, JSONBody, FormURLEncodedBody, RawBody, factories
- LCEMultipartForm: multipart builder, OutputStream-streamed serialize(),
  in-memory + on-disk file parts, ported mimeType(for:)
- Tests: StubURLProtocol harness + 15 tests

Tests/LCEssentialsTests/JSONDecoderDecodeTests.swift

@@ -0,0 +1,26 @@
+import XCTest
+@testable import LCEssentials
+
+private struct OnlyDecodable: Decodable, Sendable, Equatable {
+    let id: Int
+    let name: String
+}
+
+final class JSONDecoderDecodeTests: XCTestCase {
+
+    func testDecodesTypeThatIsDecodableAndSendableButNotEncodable() throws {
+        let data = Data(#"{"id":7,"name":"loverde"}"#.utf8)
+        let value: OnlyDecodable = try JSONDecoder.decode(data: data)
+        XCTAssertEqual(value, OnlyDecodable(id: 7, name: "loverde"))
+    }
+
+    func testDecodeFromStringOverload() throws {
+        let value: OnlyDecodable = try JSONDecoder.decode(#"{"id":1,"name":"a"}"#)
+        XCTAssertEqual(value, OnlyDecodable(id: 1, name: "a"))
+    }
+
+    func testDecodeFromURLThrowsInsteadOfCrashingOnMissingFile() {
+        let missing = URL(fileURLWithPath: "/tmp/does-not-exist-\(UUID().uuidString).json")
+        XCTAssertThrowsError(try JSONDecoder.decode(fromURL: missing) as OnlyDecodable)
+    }
+}