[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/MultipartFormTests.swift

@@ -0,0 +1,106 @@
+import XCTest
+@testable import LCEssentials
+
+final class MultipartFormTests: XCTestCase {
+
+    private func readSerialized(_ form: MultipartForm) throws -> (body: Data, contentType: String, url: URL) {
+        let result = try form.serialize()
+        let data = try Data(contentsOf: result.fileURL)
+        return (data, result.contentType, result.fileURL)
+    }
+
+    private func addCleanup(_ url: URL) {
+        addTeardownBlock { try? FileManager.default.removeItem(at: url) }
+    }
+
+    // MARK: - Fields
+
+    func testFieldPartHasNoContentTypeLine() throws {
+        var form = MultipartForm()
+        form.field("caption", "hello world")
+
+        let (body, contentType, url) = try readSerialized(form)
+        addCleanup(url)
+        let text = String(data: body, encoding: .utf8)!
+
+        XCTAssertTrue(contentType.hasPrefix("multipart/form-data; boundary="))
+        let boundary = String(contentType.split(separator: "=", maxSplits: 1)[1])
+
+        XCTAssertEqual(text, [
+            "--\(boundary)\r\n",
+            "Content-Disposition: form-data; name=\"caption\"\r\n",
+            "\r\n",
+            "hello world\r\n",
+            "--\(boundary)--\r\n"
+        ].joined())
+    }
+
+    // MARK: - In-memory data file
+
+    func testDataFilePartCarriesFilenameAndMimeAndRawBytes() throws {
+        let png = Data([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A, 0x00, 0xFF])
+        var form = MultipartForm()
+        form.file("photo", data: png, filename: "p.png")
+
+        let (body, contentType, url) = try readSerialized(form)
+        addCleanup(url)
+        let boundary = String(contentType.split(separator: "=", maxSplits: 1)[1])
+        let text = String(data: body, encoding: .isoLatin1)!
+
+        XCTAssertTrue(text.contains("Content-Disposition: form-data; name=\"photo\"; filename=\"p.png\"\r\n"))
+        XCTAssertTrue(text.contains("Content-Type: image/png\r\n"))
+        XCTAssertTrue(text.hasSuffix("--\(boundary)--\r\n"))
+
+        // raw bytes appear verbatim between the blank line and the trailing CRLF
+        let marker = Data("\r\n\r\n".utf8)
+        let range = body.range(of: marker)!
+        let afterHeader = body[range.upperBound...]
+        XCTAssertTrue(afterHeader.starts(with: png))
+    }
+
+    func testExplicitMimeOverridesGuess() throws {
+        var form = MultipartForm()
+        form.file("f", data: Data("x".utf8), filename: "a.png", mime: "application/octet-stream")
+        let (body, _, url) = try readSerialized(form)
+        addCleanup(url)
+        XCTAssertTrue(String(data: body, encoding: .utf8)!.contains("Content-Type: application/octet-stream\r\n"))
+    }
+
+    // MARK: - Disk file, streamed
+
+    func testDiskFilePartIsStreamedAndContentMatches() throws {
+        let big = Data((0..<(2 * 1024 * 1024)).map { UInt8($0 % 251) })
+        let src = FileManager.default.temporaryDirectory.appendingPathComponent("src-\(UUID().uuidString).bin")
+        try big.write(to: src)
+        addCleanup(src)
+
+        var form = MultipartForm()
+        form.field("kind", "raw")
+        form.file("doc", url: src)
+
+        let (body, contentType, out) = try readSerialized(form)
+        addCleanup(out)
+        let boundary = String(contentType.split(separator: "=", maxSplits: 1)[1])
+
+        // header uses the source filename
+        XCTAssertTrue(String(data: body.prefix(400), encoding: .isoLatin1)!
+            .contains("filename=\"\(src.lastPathComponent)\""))
+
+        // the 2 MB payload is present verbatim
+        let marker = Data("Content-Type: application/octet-stream\r\n\r\n".utf8)
+        let r = body.range(of: marker)!
+        let payload = body[r.upperBound..<(body.index(r.upperBound, offsetBy: big.count))]
+        XCTAssertEqual(Data(payload), big)
+
+        XCTAssertTrue(String(data: body.suffix(boundary.count + 8), encoding: .utf8)!
+            .hasSuffix("--\(boundary)--\r\n"))
+    }
+
+    // MARK: - MIME lookup
+
+    func testMimeTypeLookup() {
+        XCTAssertEqual(MultipartForm.mimeType(for: "a.jpg"), "image/jpeg")
+        XCTAssertEqual(MultipartForm.mimeType(for: "a.PDF"), "application/pdf")
+        XCTAssertEqual(MultipartForm.mimeType(for: "a.unknownext"), "application/octet-stream")
+    }
+}