Preserve ETags in LOCK and UNLOCK results

[Copilot]

UNLOCK responses can include a resource ETag despite containing no lock details. Expose that ETag without changing the existing optional-lock API.

• Result API

  • Add public, Sendable NKLockOperationResult with etag and lock.
  • Add lockUnlockFileResult(...) async API for both LOCK and UNLOCK.

• Compatibility

  • Keep lockUnlockFile(...) async unchanged; it delegates to the result API and returns result.lock.
  • Preserve NKLock.etag for locked responses.

• Response handling

  • Normalize quoted ETags and treat absent or empty values as nil.
  • Keep successful empty responses successful with an empty result.
  • Preserve existing request behavior and HTTP error propagation.

let result = try await kit.lockUnlockFileResult(
    serverUrlFileName: fileURL,
    shouldLock: false,
    account: account
)

let etag = result.etag
let lock = result.lock // nil after UNLOCK

• Coverage
  • Add mocked LOCK, UNLOCK, empty-body, missing-ETag, quoted-ETag, HTTP-failure, and legacy-API tests.