[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/Support/StubURLProtocolTests.swift

@@ -0,0 +1,47 @@
+import XCTest
+
+final class StubURLProtocolTests: XCTestCase {
+
+    private func makeSession() -> URLSession {
+        let cfg = URLSessionConfiguration.ephemeral
+        cfg.protocolClasses = [StubURLProtocol.self]
+        return URLSession(configuration: cfg)
+    }
+
+    override func tearDown() {
+        StubURLProtocol.reset()
+        super.tearDown()
+    }
+
+    func testReplaysCannedResponseAndCapturesRequest() async throws {
+        var stub = StubURLProtocol.Stub()
+        stub.statusCode = 201
+        stub.body = Data(#"{"ok":true}"#.utf8)
+        StubURLProtocol.setStub(stub)
+
+        var request = URLRequest(url: URL(string: "https://example.com/things")!)
+        request.httpMethod = "POST"
+        request.httpBody = Data(#"{"name":"x"}"#.utf8)
+
+        let (data, response) = try await makeSession().data(for: request)
+
+        XCTAssertEqual((response as? HTTPURLResponse)?.statusCode, 201)
+        XCTAssertEqual(String(data: data, encoding: .utf8), #"{"ok":true}"#)
+        XCTAssertEqual(StubURLProtocol.requestCount, 1)
+        XCTAssertEqual(StubURLProtocol.capturedRequests.first?.httpMethod, "POST")
+        XCTAssertEqual(StubURLProtocol.lastCapturedBody, Data(#"{"name":"x"}"#.utf8))
+    }
+
+    func testReplaysError() async {
+        var stub = StubURLProtocol.Stub()
+        stub.error = URLError(.notConnectedToInternet)
+        StubURLProtocol.setStub(stub)
+
+        do {
+            _ = try await makeSession().data(for: URLRequest(url: URL(string: "https://example.com")!))
+            XCTFail("expected throw")
+        } catch {
+            XCTAssertEqual((error as? URLError)?.code, .notConnectedToInternet)
+        }
+    }
+}