Replace todo!() panics with structured errors for unsupported ScAddress types in signer

[Rachit2323]

What:
Three ScAddress variants (MuxedAccount, ClaimableBalance, LiquidityPool) in
signer/mod.rs were hitting todo!() macros instead of returning proper errors.
Replaced them with structured Error variants and added a unit test.

Why:
If a Soroban auth entry contains one of these address types, the CLI crashes
with a panic instead of showing a useful message. This is a bad experience —
users see a raw thread panic with no guidance. Now they get a clear error and
the process exits cleanly.

Known limitations:
MuxedAccount signing is not implemented yet — this just makes the failure
graceful. Full support can be added in a follow-up.

[Copilot]

Pull request overview

This PR improves Soroban authorization signing in soroban-cli by replacing todo!() panics for unsupported ScAddress variants with structured Error returns, preventing CLI crashes and providing clearer user-facing failures.

Changes:

• Added explicit Error variants for unsupported ScAddress types (MuxedAccount, ClaimableBalance, LiquidityPool).
• Updated sign_soroban_authorizations to return those errors instead of panicking.
• Added a unit test covering the MuxedAccount unsupported-path behavior.

Comments suppressed due to low confidence (1)

cmd/soroban-cli/src/signer/mod.rs:111

• The ScAddress::Contract(...) branch returns Error::MissingSignerForAddress even though the comment states contract addresses aren't supported, and there is already an Error::ContractAddressAreNotSupported { .. } variant. This yields a misleading error message ("Missing signing key for account...") for contract addresses. Consider returning ContractAddressAreNotSupported here (or removing that unused variant if it's not intended to be used).

            ScAddress::Contract(stellar_xdr::curr::ContractId(Hash(c))) => {
                // This address is for a contract. This means we're using a custom
                // smart-contract account. Currently the CLI doesn't support that yet.
                return Err(Error::MissingSignerForAddress {
                    address: stellar_strkey::Strkey::Contract(stellar_strkey::Contract(*c))
                        .to_string(),
                });
            }