Use expirationDurationInSeconds for token refresh window

[kylebrowning]

Problem

APNSAuthenticationTokenManager declares expirationDurationInSeconds = .seconds(60 * 55) but never reads it — the 55-minute refresh window is hardcoded inline in nextValidToken. Changing the named constant would silently have no effect, a latent foot-gun.

Separately, testTokenIsRefreshed asserts only that two token strings differ — but ECDSA P256 signatures are randomized, so the strings differ even when the payload is unchanged. The test passes for the wrong reason and proves little about the refresh.

Changes

• Reference expirationDurationInSeconds in the refresh check so it is the single source of truth.
• testTokenReusedJustBeforeBoundaryAndRefreshedAtBoundary pins the [0, 55min) reuse window at the exact boundary (reused at 54:59, refreshed at 55:00).
• testRefreshedTokenIsStructurallyValid decodes the refreshed JWT and asserts the header/claims (alg, kid, iss), instead of relying on string inequality.

Note

I deliberately did not change the JWT iat from DispatchWallTime to the injected Clock: ContinuousClock is monotonic and cannot yield epoch seconds, so wall-clock time is the correct source for iat, while the injected clock correctly measures the refresh interval.

Testing

swift test --filter APNSAuthenticationTokenManagerTests — 5 tests pass.