[api-upload-refactor] Add multipart upload(), split logging to its own file

- upload<T>(url:method:form:pathParams:headers:debug:timeoutInterval:networkServiceType:)
  serialises MultipartForm to a temp file, streams it via uploadTask(with:fromFile:)
  bridged to async (works on iOS 13+, unlike URLSession.upload(for:fromFile:))
- temp body file always removed via defer (success and throw)
- move requestLOG/responseLOG to LCEssentials+API+Logging.swift; extract
  logResponseError helper; keeps API file at 473 lines
- APIUploadTests: 4 cases
- progress-reporting overload deferred (no stub seam for didSendBodyData)

Tests/LCEssentialsTests/APIUploadTests.swift

@@ -0,0 +1,95 @@
+import XCTest
+@testable import LCEssentials
+
+private struct UploadEcho: Decodable, Sendable, Equatable {
+    let ok: Bool
+}
+
+final class APIUploadTests: XCTestCase {
+
+    private var api: API!
+
+    override func setUp() {
+        super.setUp()
+        let cfg = URLSessionConfiguration.ephemeral
+        cfg.protocolClasses = [StubURLProtocol.self]
+        api = API(testConfiguration: cfg)
+    }
+
+    override func tearDown() {
+        StubURLProtocol.reset()
+        api = nil
+        super.tearDown()
+    }
+
+    private func tempCountInDir(_ dir: URL) -> Int {
+        (try? FileManager.default.contentsOfDirectory(atPath: dir.path).filter {
+            $0.hasPrefix("lce-multipart-")
+        }.count) ?? -1
+    }
+
+    func testUploadSendsMultipartBodyAndDecodesResponse() async throws {
+        StubURLProtocol.setStub(.init(statusCode: 200, body: Data(#"{"ok":true}"#.utf8)))
+
+        var form = MultipartForm()
+        form.field("caption", "hi")
+        form.file("photo", data: Data([0x89, 0x50, 0x4E, 0x47]), filename: "p.png")
+        let expectedBody = try Data(contentsOf: form.serialize().fileURL)
+
+        let result: UploadEcho = try await api.upload(
+            url: "https://api.example.com/media",
+            form: form
+        )
+
+        XCTAssertEqual(result, UploadEcho(ok: true))
+        let sent = StubURLProtocol.capturedRequests.first
+        XCTAssertEqual(sent?.httpMethod, "POST")
+        XCTAssertTrue(sent?.value(forHTTPHeaderField: "Content-Type")?
+            .hasPrefix("multipart/form-data; boundary=") ?? false)
+        // body framing matches a fresh serialize() (boundary differs per form
+        // instance, so compare structure, not bytes, by re-serialising the SAME form)
+        XCTAssertEqual(StubURLProtocol.lastCapturedBody?.count, expectedBody.count)
+    }
+
+    func testTempBodyFileRemovedAfterSuccess() async throws {
+        StubURLProtocol.setStub(.init(statusCode: 200, body: Data(#"{"ok":true}"#.utf8)))
+        let dir = FileManager.default.temporaryDirectory
+        let before = tempCountInDir(dir)
+
+        var form = MultipartForm()
+        form.file("f", data: Data("x".utf8), filename: "a.txt")
+        let _: UploadEcho = try await api.upload(url: "https://api.example.com/x", form: form)
+
+        XCTAssertEqual(tempCountInDir(dir), before)
+    }
+
+    func testTempBodyFileRemovedAfterThrow() async {
+        StubURLProtocol.setStub(.init(statusCode: 500, body: Data(#"{"error":"boom"}"#.utf8)))
+        let dir = FileManager.default.temporaryDirectory
+        let before = tempCountInDir(dir)
+
+        var form = MultipartForm()
+        form.file("f", data: Data("x".utf8), filename: "a.txt")
+        do {
+            let _: UploadEcho = try await api.upload(url: "https://api.example.com/x", form: form)
+            XCTFail("expected throw")
+        } catch {
+            // expected
+        }
+        XCTAssertEqual(tempCountInDir(dir), before)
+    }
+
+    func testUploadServerErrorThrowsWithStatus() async {
+        StubURLProtocol.setStub(.init(statusCode: 413, body: Data(#"{"error":"too big"}"#.utf8)))
+        var form = MultipartForm()
+        form.file("f", data: Data("x".utf8), filename: "a.txt")
+        do {
+            let _: UploadEcho = try await api.upload(url: "https://api.example.com/x", form: form)
+            XCTFail("expected throw")
+        } catch let error as NSError {
+            XCTAssertEqual(error.code, 413)
+        } catch {
+            XCTFail("wrong error: \(error)")
+        }
+    }
+}