huawei-push-kit: use expires_in from responses

[pjfanning]

1. The old behavior was a bug, not a feature — hardcoding 3600s when the server explicitly tells you otherwise is incorrect per OAuth2 spec. A config to "revert to wrong" is unusual.
2. If the server returns bad data, that's a different problem — and the right fix for that would be validation/fallback logic (e.g., clamp to a reasonable range), not a toggle to ignore the server entirely.
3. Config surface area — every config option is a maintenance and documentation burden. This one would be hard to name clearly and would confuse users.

What might make more sense as a safety net is a buffer — subtract a small amount (e.g., 60s) from expires_in to account for clock skew and network latency, which is a common pattern:

expiresAt = now + result.expires_in - 60

This protects against edge cases without adding config.