Payment errors collapse insufficient-funds into generic Error::Payment(String) — add a typed InsufficientFunds variant

[Nic-dorman]

Summary

On the wallet-backed payment path, ant-core throws away a perfectly good typed error from evmlib and stringifies it into the generic Error::Payment(String), so downstream consumers (the mobile FFI / SDK, the CLI) can't distinguish "not enough ANT tokens" from any other payment failure without brittle substring matching.

Where the information is lost

evmlib already exposes a distinct variant and pre-checks the balance before submitting:

• evmlib/src/wallet.rs:35 — Error::InsufficientTokensForQuotes(Amount, Amount) (have, need)
• pay_for_quotes() and pay_for_merkle_tree() both pre-check balance_of_tokens() and return InsufficientTokensForQuotes(have, need) before sending any tx.

ant-core then flattens it:

• ant-core/src/data/client/payment.rs:92-94 — .map_err(|e| Error::Payment(format!("On-chain payment failed: {e}")))
• ant-core/src/data/client/payment.rs:126-129 — token approval, same pattern
• ant-core/src/data/client/merkle.rs:488-495 — .map_err(|e| Error::Payment(format!("Merkle batch payment failed: {e}")))

Ask

1. Add a typed variant to ant-core/src/data/error.rs, e.g.:

   #[error("insufficient funds: have {have} atto, need {need} atto")]
   InsufficientFunds { have: String, need: String },

2. In payment.rs / merkle.rs, match evmlib::wallet::Error::InsufficientTokensForQuotes(have, need) (reached via ant_protocol::evm) and map it to the new variant before the generic stringify fallback.

This lets consumers surface a clean "insufficient funds — top up your wallet" instead of a generic payment error.

Out of scope / follow-ups (noted for context)

• ETH-for-gas shortfall is not pre-checked; it comes back as an RPC "insufficient funds for gas", and evmlib/src/retry.rs:353 extract_revert_data() currently always returns None, so the revert reason is dropped. Distinguishing gas shortfall would need evmlib to populate revert data (separate, larger change).
• The external-signer path (mobile WalletConnect) never routes payment through ant-core, so this variant doesn't apply there — that path is handled SDK-side with a read-only pre-flight balance check.

Downstream

Consumed by the mobile SDK error mapping in WithAutonomi/ant-sdk (ffi/rust/ant-ffi/src/lib.rs From<ant_core::data::Error>), tracked on Linear V2-601. Once this lands, the FFI adds a ClientError::InsufficientFunds variant.