[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/HTTPBodyTests.swift

@@ -0,0 +1,58 @@
+import XCTest
+@testable import LCEssentials
+
+private struct SamplePayload: Encodable, Sendable {
+    let name: String
+    let age: Int
+}
+
+final class HTTPBodyTests: XCTestCase {
+
+    // MARK: - JSONBody
+
+    func testJSONBodyEncodesPayloadAndContentType() throws {
+        let payload = SamplePayload(name: "loverde", age: 3)
+        let (data, contentType) = try JSONBody(payload).encoded()
+
+        XCTAssertEqual(contentType, "application/json; charset=UTF-8")
+
+        let decoded = try JSONDecoder().decode([String: AnyDecodable].self, from: data)
+        XCTAssertEqual(decoded["name"]?.value as? String, "loverde")
+        XCTAssertEqual(decoded["age"]?.value as? Int, 3)
+    }
+
+    func testJSONBodyFactory() throws {
+        let (data, _) = try jsonBody(SamplePayload(name: "x", age: 1)).encoded()
+        XCTAssertFalse(data.isEmpty)
+    }
+
+    // MARK: - FormURLEncodedBody
+
+    func testFormURLEncodedEncodesPairsAndPercentEscapes() throws {
+        let (data, contentType) = try FormURLEncodedBody(["a": "1", "b": "two words"]).encoded()
+
+        XCTAssertEqual(contentType, "application/x-www-form-urlencoded; charset=UTF-8")
+
+        let pairs = Set(String(data: data, encoding: .utf8)!.split(separator: "&").map(String.init))
+        XCTAssertEqual(pairs, ["a=1", "b=two%20words"])
+    }
+
+    func testFormURLEncodedEscapesReservedCharactersInsteadOfDropping() throws {
+        let (data, _) = try FormURLEncodedBody(["q": "a&b=c"]).encoded()
+        let body = String(data: data, encoding: .utf8)!
+        XCTAssertEqual(body, "q=a%26b%3Dc")
+    }
+}
+
+/// Minimal type-erased decoder for asserting JSON shape in tests.
+struct AnyDecodable: Decodable {
+    let value: Any
+    init(from decoder: Decoder) throws {
+        let c = try decoder.singleValueContainer()
+        if let i = try? c.decode(Int.self) { value = i }
+        else if let s = try? c.decode(String.self) { value = s }
+        else if let b = try? c.decode(Bool.self) { value = b }
+        else if let d = try? c.decode(Double.self) { value = d }
+        else { value = "" }
+    }
+}