feat(abstract-eth): expose getSignablePayload for ETH coin classes (CGD-1083)

[0xPrabh]

Description

Overrides getSignablePayload(serializedTx) on AbstractEthLikeCoin so that ETH and all ETH-like coins return the correct 32-byte keccak256 signing digest, rather than falling through to the base-class no-op that returns raw serialized bytes.

This is required to unblock AKM's external signing endpoint (POST /sign), which needs the correct hash to sign — not the raw transaction bytes.

Changes

• AbstractEthLikeCoin.getSignablePayload — new override: deserializes via the transaction builder, casts to EthTransaction, and returns tx.signablePayload
• EthLikeTransactionData interface — adds getSignablePayload(): Buffer
• EthTransactionData — implements getSignablePayload() via tx.getMessageToSign(true) (keccak256 hash, same primitive used internally for TSS signing)
• Transaction.signablePayload getter — overrides BaseTransaction's NotImplementedError default; throws InvalidTransactionError if no transaction data is set
• EthLikeToken.getSignablePayload — preserves pre-PR behavior (returns raw serialized bytes); tokens are out of scope for this ticket
• Inherited by all non-token AbstractEthLikeNewCoins descendants (Eth, Polygon, Arbeth, Opeth, BSC, AvaxC, EVM, etc.) and legacy AbstractEthLikeCoin descendants (Rbtc, Celo, Etc)

Issue

CGD-1083 (unblocks WCN-397)

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Unit tests added in:

• modules/abstract-eth/test/unit/transaction.ts — throws on empty tx; returns 32-byte hash for unsigned tx
• modules/sdk-coin-eth/test/unit/transaction.ts — hash pinned against txData.id fixture for both Legacy and EIP1559; distinct-payload check between tx types
• modules/sdk-coin-eth/test/unit/eth.ts — end-to-end coin.getSignablePayload(serializedTx) for Legacy and EIP1559 via Teth

Existing TSS signing flows are unaffected — they use signableHex from the unsigned tx object directly (abstractEthLikeNewCoins.ts:2444), not getSignablePayload.

[linear-code]

CGD-1083

[venkateshv1266]

🔴 Token coins throw on getSignablePayload — contradicts PR description

modules/abstract-eth/src/ethLikeToken.ts:406-408: EthLikeToken.getTransactionBuilder() is stubbed to throw new Error('Method not implemented'). Because EthLikeToken extends AbstractEthLikeNewCoins extends AbstractEthLikeCoin, it inherits the new override at abstractEthLikeCoin.ts:234-239, whose first line is this.getTransactionBuilder(). Calling <token>.getSignablePayload(serializedTx) now throws 'Method not implemented'.

Why this blocks: The PR description explicitly lists tokens as inheritors of this behavior ("Inherited by all AbstractEthLikeNewCoins descendants (Eth, Polygon, Arbeth, Opeth, BSC, AvaxC, EVM, tokens, etc.)"). Today the base-class default at least returned a Buffer (broken UTF-8, but non-throwing); now any ERC-20 path through AKM's /sign will hard-fail with an opaque error.

Suggested fix: Either

• (a) override getSignablePayload on EthLikeToken to construct a vanilla ETH-coin builder — the wire-level tx is just an ETH tx, the token semantics live in the calldata; or
• (b) explicitly throw a clear NotImplementedError so callers can fall back, and update the PR description to exclude tokens.

In either case, please add a token unit test mirroring the new eth.ts end-to-end tests (e.g. tdai) so this regression is caught by CI.