fix(sdk-coin-eth): complete sepeth registration and fix testnet coin resolution

[rithwik-bitgo]

Summary

• Register sepeth (Sepolia Ethereum) as an SDK coin — was missing, blocking Sepolia wallet creation (follow-up to feat(statics): add sepolia testnet for ETH #9377, feat(statics): give Sepolia testnet its own underlying asset (sepETH) #9389).
• Add Networks.test.sepolia → sepeth mapping in getErc20TokenConfig() and getErc7984TokenConfig() in statics.
• Fix base coin resolution for Eth-family testnet tokens in erc20Token.ts/erc7984Token.ts:
  • Erc20Token/Erc7984Token picked their base staticsCoin via coinNames[tokenConfig.network], where coinNames = { Mainnet: 'eth', Testnet: 'hteth' }. Since tokenConfig.network is only ever the generic bucket 'Mainnet' or 'Testnet' (never the specific chain), every testnet token — Holesky, Hoodi, or Sepolia — resolved to the same fixed 'hteth'.
  • This worked by coincidence while hteth was the only Eth testnet with live tokens. With sepeth now active, a Sepolia token would have silently built/signed transactions using hteth's chain data (chainId 17000 instead of 11155111, wrong wallet/forwarder contract addresses).
  • Fix: resolve staticsCoin from tokenConfig.coin instead, which is already set per-network by getErc20TokenConfig()/getErc7984TokenConfig() ('hteth', 'sepeth', etc.).
  • Mainnet is unaffected — tokenConfig.coin is 'eth' for mainnet tokens, same value the old coinNames.Mainnet lookup produced.

Test plan

• yarn tsc --noEmit clean for sdk-coin-eth and statics
• sdk-coin-eth full unit suite passing (382 tests)
• statics tokenConfig-related tests passing
• Updated register.ts unit test to account for new sepeth base coin registration

Ticket: CECHO-1721

[linear-code]

CECHO-1721

[manojkumar138]

@claude review

[github-actions]

Claude finished @manojkumar138's task —— View job

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Code Review

Todo List:

• Read repository guidelines and setup
• Review core logic changes in erc20Token.ts and erc7984Token.ts
• Examine new sepeth.ts implementation
• Check registration updates in register.ts
• Review test changes
• Analyze tokenConfig.ts modifications
• Provide comprehensive review feedback

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Summary

This PR successfully addresses the missing Sepolia Ethereum (sepeth) registration and fixes a critical bug in testnet token base coin resolution. The changes are well-structured and solve the exact problems described in the PR description.

Key Changes Analysis

✅ Core Logic Fix (erc20Token.ts & erc7984Token.ts)

Issue Fixed: The original code used Erc20Token.coinNames[tokenConfig.network] where tokenConfig.network is always generic ('Mainnet' or 'Testnet'), causing all testnet tokens to incorrectly resolve to 'hteth' regardless of their actual network.

Solution: Changed to coins.get(tokenConfig.coin) where tokenConfig.coin is already set per-network by getErc20TokenConfig()/getErc7984TokenConfig() ('hteth', 'sepeth', etc.).

This is a critical fix - without it, Sepolia tokens would have used Holesky chain data (chainId 17000 instead of 11155111, wrong contract addresses).

✅ Sepeth Implementation (sepeth.ts)

The new sepeth.ts follows the established pattern:

• Extends Eth base class ✓
• Implements createInstance factory method ✓
• Clean, minimal implementation matching other testnet coins ✓

✅ Registration Updates (register.ts)

• Properly imports Sepeth class
• Registers sepeth in both register() and registerWithCoinMap() functions
• Maintains alphabetical ordering of registrations

✅ Configuration Updates (tokenConfig.ts)

• Adds Networks.test.sepolia.name → 'sepeth' mapping in both getErc20TokenConfig() and getErc7984TokenConfig()
• Maintains consistency with existing patterns
• Provides proper network-to-coin resolution

✅ Test Updates

• Updates unit test expectations to account for new sepeth registration
• Correctly increments expected call count from 4 to 5 base coins
• Verifies sepeth is properly registered in both test scenarios

Security & Architecture Considerations

1. No Security Issues: The changes don't introduce any security vulnerabilities
2. Backwards Compatible: Mainnet functionality unchanged, only fixes testnet behavior
3. Type Safety: All changes maintain proper TypeScript typing
4. Pattern Consistency: New code follows established architectural patterns

Minor Observations

1. Static coinNames Property: The coinNames property in both token classes is now unused after the fix. Consider removing it in a future cleanup PR.

2. Documentation: Consider updating any documentation that might reference the old behavior, though none appear to be affected in this changeset.

Test Coverage

The PR author mentions running:

• TypeScript compilation checks ✓
• Full sdk-coin-eth unit suite (382 tests) ✓
• statics tokenConfig tests ✓
• Updated register.ts unit test ✓

Recommendation: APPROVE ✅

This PR:

• ✅ Fixes a critical bug that would cause incorrect chain data usage for Sepolia tokens
• ✅ Properly implements missing sepeth registration
• ✅ Follows established code patterns and conventions
• ✅ Includes appropriate test updates
• ✅ Has no breaking changes or security issues
• ✅ Solves the exact problems described in the Linear ticket

The implementation is clean, well-tested, and addresses a blocking issue for Sepolia wallet creation while preventing silent failures that could lead to transaction errors.

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Job run: https://github.com/BitGo/BitGoJS/actions/runs