fix(key-wallet): check child indexes in the dip9 path builders #1297

DerivationPath::bip_44_payment_path (and the other dip9.rs path builders) construct ChildNumber::Hardened { index } and ChildNumber::Normal { index } directly from caller u32s, without the 31-bit check that ChildNumber::from_hardened_idx / from_normal_idx apply.

An account of 0x8000_0000 therefore builds the same path as account 0, because a hardened child serializes as index | 1 << 31. An address_index of 0x8000_0000 or more builds a Normal child that is not canonical. DerivationPath::from_str and AccountType::derivation_path reject both values, so the same request gives different answers depending on which constructor a caller uses.

Found in review of dashpay/platform#5727, whose platform_wallet_bip44_address_from_mnemonic passes user-supplied indexes to bip_44_payment_path.

Proposed fix: make the path builders fallible (from_hardened_idx / from_normal_idx) and return the error, then cover 0x7fff_ffff against 0x8000_0000 in tests. Callers propagate the error instead of adding their own range checks.