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

[alextse-bg]

some nits