[api-upload-refactor] Convert API to an actor, drop @MainActor

- API: @MainActor struct -> actor. certData/certPassword/persistConnectionDelay
  move from static var (data race) to actor-isolated instance state
- setupCertificationRequest -> setupCertification; add setPersistConnectionDelay
- add internal init(testConfiguration:) seam; makeSession() helper picks
  test / cert-delegate / shared session
- URLSessionDelegateHandler: final + @unchecked Sendable, immutable let certs,
  no @MainActor; challenge logic unchanged
- getIdentity(): remove as! SecIdentity, extract via typed Unmanaged bridging
- delete unused Result enum and defaultParams
- APIActorTests: actor conformance, delay round-trip, per-instance cert state

Tests/LCEssentialsTests/APIActorTests.swift

@@ -0,0 +1,26 @@
+import XCTest
+@testable import LCEssentials
+
+final class APIActorTests: XCTestCase {
+
+    func testAPIIsAnActorNotMainActorBound() async {
+        let isActor = (API.shared as Any) is any Actor
+        XCTAssertTrue(isActor, "API must be an actor so callers are not forced onto the main thread")
+    }
+
+    func testPersistConnectionDelayRoundTrips() async {
+        let api = API(testConfiguration: .ephemeral)
+        await api.setPersistConnectionDelay(9)
+        let value = await api.persistConnectionDelay
+        XCTAssertEqual(value, 9)
+    }
+
+    func testIsolatedTestInstanceDoesNotTouchSharedCertState() async {
+        let api = API(testConfiguration: .ephemeral)
+        await api.setupCertification(certData: Data([0x01, 0x02]), password: "pw")
+        let sharedHasCert = await API.shared.hasClientCertificateConfigured
+        let instanceHasCert = await api.hasClientCertificateConfigured
+        XCTAssertFalse(sharedHasCert)
+        XCTAssertTrue(instanceHasCert)
+    }
+}