Return typed APNSError for undecodable error bodies and add broadcast error reasons

[kylebrowning]

Replaces #247 (closed during a base-branch cleanup).

• The NIO clients threw a raw DecodingError/NIOTooManyBytesError when an APNs error body was empty, non-JSON, or oversized — losing the status code. They now return a typed APNSError with reason: nil, matching the URLSession client.
• Adds Apple's 9 documented broadcast/channel error reasons (FeatureNotEnabled, MissingChannelId, BadChannelId, ChannelNotRegistered, BadRequestParams, BadRequestPayload, MissingPushType, CannotCreateChannelConfig, TopicMismatch).
• Deprecates badEnvironmentKeyInToken (not a documented APNs reason); fixes the timestamp doc (accompanies any 410, not just Unregistered); apnsUniqueID is now let.
• Test server: read/delete of a missing channel now returns Apple's documented 400 ChannelNotRegistered instead of a made-up 404.
• New error-path tests (forced 500/503/429, empty/HTML/oversized bodies) via the server's response-override hook.