[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
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