key-wallet: `from_account` / `from_bls_account` / `from_eddsa_account` panic on unsupported AccountType

[QuantumExplorer]

Context

Surfaced by CodeRabbit during review of #711. The panic paths are pre-existing (inherited from the original `ManagedCoreAccount::from_*_account` constructors that #711 split). Both `ManagedCoreFundsAccount` and `ManagedCoreKeysAccount` carry the same `expect(...)` fallback today.

Repo coding guideline: "Avoid `unwrap()` and `expect()` in library code; use proper error types (e.g. `thiserror`) and never panic in library code."

Problem

The constructors look like:

pub fn from_account(account: &super::super::Account) -> Self {
    let key_source = address_pool::KeySource::Public(account.account_xpub);
    let managed_type = ManagedAccountType::from_account_type(
        account.account_type,
        account.network,
        &key_source,
    )
    .unwrap_or_else(|_| {
        let no_key_source = address_pool::KeySource::NoKeySource;
        ManagedAccountType::from_account_type(
            account.account_type,
            account.network,
            &no_key_source,
        )
        .expect("Should succeed with NoKeySource")  // <-- panics
    });
    Self::new(managed_type, account.network, account.is_watch_only)
}

If `from_account_type` errors even with `NoKeySource` (e.g. an account variant we forgot to handle), the constructor aborts the caller's process instead of returning a structured error.

Suggested fix

Convert all three constructors (`from_account`, `from_bls_account`, `from_eddsa_account`) on both `ManagedCoreFundsAccount` and `ManagedCoreKeysAccount` to return `Result<Self, Error>` and propagate the underlying `crate::error::Error` from `ManagedAccountType::from_account_type`.

Touches every callsite, including the FFI surface in `key-wallet-ffi`. Worth doing in one focused PR.

Files

• `key-wallet/src/managed_account/managed_core_funds_account.rs`
• `key-wallet/src/managed_account/managed_core_keys_account.rs`
• (call sites discovered via `cargo check`)

Original review thread: #711 (review comment 3187368652)