feat(relay): error + requireFunds capabilities

[jxom]

Changes

Opt-in feature gating

Relay features are now opt-in via the features option on Handler.relay. By default, only fee payer sponsorship is active.

Set features: 'all' to enable everything:

const handler = Handler.relay({
  features: 'all',
  feePayer: {
    account: privateKeyToAccount('0x...'),
  },
})

This enables:

• Fee token resolution — picks the best fee token by balance
• Auto-swap — AMM swaps on insufficient fee token balance
• Simulation — balance diffs + fee breakdown in capabilities

Individual overrides still work alongside features: 'all':

• autoSwap: false disables auto-swap
• autoSwap: { slippage: 0.02 } customizes slippage
• resolveTokens enables fee token resolution independently

ExecutionError parser

New ExecutionError module that parses Tempo revert errors into structured, human-readable error reasons.

requireFunds capability

When eth_fillTransaction fails with InsufficientBalance, the relay returns a requireFunds capability with the deficit amount, token, and metadata.

Other

• Updated relay tests to enable features where exercised.
• Dependency bumps.