fix(mobile): keep push discovery working when NIP-11 grows

[superkim0610]

Summary

iOS push never activates against current relays. BuzzPushLeaseDescriptor.fromRelayInformation rejected any NIP-11 document with a top-level field outside a fixed allowlist. The relay always emits artifacts and also emits read_state_snapshot (#7572), so every push-enabled relay failed that check. Discovery treats this as "no push capability" and only logs it with debugPrint. The app never asks for notification permission, never registers with APNs, and never enrolls or publishes a lease, so turning push on in Settings does nothing. The pending lease revocation outbox calls the same function, so it was blocked as well.

NIP-11 is an extensible document. NIP-PL requires strict validation only for the push descriptor. This removes the top-level allowlist and leaves descriptor validation unchanged. Unknown fields inside push are still rejected, as covered by descriptor rejects unknown push fields.

Related issue

None found. Companion fixes for iOS push end to end:

• fix(push-gateway): accept App Attest assertions with extension data #8020: the gateway rejects iOS 27 App Attest assertions.
• fix(mobile/ios): play the default sound for push notifications #8021: notifications have no sound.

Testing

• The regression test descriptor ignores relay metadata outside push adds artifacts, read_state_snapshot and an invented future field. Before the fix it fails with FormatException: NIP-11 document contains unknown field artifacts, the same error a device hits. After the fix it passes.
• flutter test --dart-define=BUZZ_PUSH_GATEWAY_URL=https://push.example test/shared/push/ passes 99 tests with 3 skipped. dart format and flutter analyze are clean.
• Device check by a human: an iPhone on iOS 27 ran a TestFlight build with this fix against a self-hosted relay (ghcr.io/block/buzz:main, BUZZ_PUSH_ENABLED=true). The notification permission prompt appeared and the gateway enrollment started. A stock build never prompted.
• Agent review: READY, no blockers. I applied its optional test-strengthening suggestion.

buzz-review-completed

[github-actions]

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