Fix payload encoding for VoIP, PushToTalk, Complication, FileProvider, and Location notifications

[kylebrowning]

These five notification types had no custom encode(to:), so the synthesized Encodable sent header metadata (topic, expiration, priority, apnsID) in the HTTP body and nested the user payload under "payload" instead of the root — contradicting each type's own documentation.

• VoIP/PushToTalk/Complication/FileProvider now encode only the user payload at the root (same pattern as APNSBackgroundNotification).
• Location (which has no payload) now sends a minimal {"aps":{}} body; Apple documents no body shape for location pushes.
• Adds exact-JSON encoding tests for all five types, plus the previously missing Location topic-suffix test.