Add Support for MegaETH

This PR adds full support for MegaETH (Chain.MegaETH) to the opensea-js

The integration follows the existing multi-chain architecture and mirrors the implementation pattern used for other alternate-protocol chains such as Gunzilla and Somnia.

No breaking changes were introduced.

Changes Included

1. Chain Integration

• Added Chain.MegaETH
• Added chain ID mapping:

case Chain.MegaETH:
  return "4326";

2. Alternate Protocol Routing

MegaETH uses the alternate Seaport deployment.

Updated:

export const usesAlternateProtocol = (chain: Chain): boolean =>
  chain === Chain.Gunzilla || chain === Chain.Somnia || chain === Chain.MegaETH;

As a result, MegaETH now correctly resolves to:

• ALTERNATE_SEAPORT_V1_6_ADDRESS
• ALTERNATE_CONDUIT_ADDRESS
• ALTERNATE_SIGNED_ZONE_V2_ADDRESS

via:

• getSeaportAddress
• getDefaultConduit
• getSignedZone

3. Fee Recipient

Added a dedicated fee recipient constant:

export const MEGAETH_FEE_RECIPIENT =
  "0x07D3A100c3880830dD43FE5C938B5144721Ce9D6";

Integrated inside getFeeRecipient into:

case Chain.MegaETH:
  return MEGAETH_FEE_RECIPIENT;

4. Offer Payment Token

MegaETH uses the standard OP-stack wrapped ETH address:

0x4200000000000000000000000000000000000006

Added to:

• getOfferPaymentToken
• getNativeWrapTokenAddress (via default flow)

5. Listing Payment Token

Listings on MegaETH use native ETH:

0x0000000000000000000000000000000000000000

Integrated into getListingPaymentToken.

6. Test Coverage

Added basic test cases in chain.spec.ts covering:

• Chain ID resolution
• Alternate protocol detection
• Fee recipient selection
• Default offer and listing token behavior

All tests pass successfully.

Architectural Consistency

The implementation:

• Follows the existing switch-case structure
• Reuses shared alternate protocol constants
• Keeps fee logic isolated per chain
• Maintains backward compatibility