chore(contract): sync flipcash2 protos to 9ebf55fe

[bmc08gt]

Syncs proto/ to flipcash2-protobuf-api@9ebf55fe (upstream #117 to #122) and adds the 0.12.0 changelog entry. Most of it is end-to-end encryption for DMs.

• messaging.v1: EncryptedContent as Content.encrypted = 7 (scheme, 24-byte nonce, ciphertext); ENCRYPTION_NOT_ALLOWED appended to SendMessageResponse.Result (= 2) and EditMessageResponse.Result (= 5).
• blob.v1: InitiateExternalUploadRequest.end_to_end_encrypted_for { chat = 4 }, BlobMetadata.encrypted = 5 (EncryptedBlobMetadata), UploadPolicy.encrypted (EncryptedConstraints).
• chat.v1.Metadata: creator (13) and use_e2ee (100; Swift accessor useE2Ee).
• push.v1.ChatMetadata: message (3) moves into a message_ref oneof with the new message_id (5). Wire-compatible, but Swift loses hasMessage / clearMessage(), and a push for a long message carries only message_id.

No field or enum case was renumbered; both new Result cases are appended after the last existing one, so iOS's positional rawValue mapping doesn't shift.

Release as 0.12.0 after merge. App PRs pinning it (drafts until it's published):

• feat(chat): handle the E2EE contract additions and bump the contract packages code-android-app#1573
• feat(chat): handle the E2EE contract additions and bump the contract packages code-ios-app#870

The ocp sync ships alongside: code-payments/ocp-client-protocol#9