fix(express): restore backward compatibility for empty eip1559 object

[Marzooqa]

Summary

• Restores backward compatibility for clients sending eip1559: {}
• Adds validation to reject partial eip1559 objects (only one of
  maxFeePerGas/maxPriorityFeePerGas)
• Applies to both /sendcoins and /sendmany endpoints

Changes

• Updated EIP1559Params schema to use t.partial for flexible input validation
• Added controller-level validation with clear error messages:
  • eip1559 missing maxPriorityFeePerGas
  • eip1559 missing maxFeePerGas
• Empty eip1559: {} is normalized to undefined.
• Added tests for backward compatibility and partial object rejection

TICKET: WP-7882

[lokesh-bitgo]

Have you confirmed whether both fields are required to be passed here?

[Marzooqa]

Yes. The server-side auto-estimation only triggers when both fields are missing. If only one field is provided, the server won't auto-estimate, it will fail downstream. Better to reject early with a clear error.

[alextse-bg]

nit:

export const EIP1559Params = t.union([
  t.type({}),
  t.type({
    /** Maximum priority fee per gas (in wei) - REQUIRED */
    maxPriorityFeePerGas: t.union([t.number, t.string]),
    /** Maximum fee per gas (in wei) - REQUIRED */
    maxFeePerGas: t.union([t.number, t.string]),
  })
]);

[alextse-bg]

some nits