feat: add zenchain testnet

[LordMarkDev]

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PR-Codex overview

This PR introduces the Zenchain Testnet to the project, enhancing the chain definitions with its details and enabling its export for use in other parts of the application.

Detailed summary

• Added zenchainTestnet export in src/chains/index.ts.
• Created zenchainTestnet definition in src/chains/definitions/zenchainTestnet.ts with:
  • ID: 8408
  • Native currency: ZCX (18 decimals)
  • RPC URLs for HTTP and WebSocket
  • Multicall3 contract details
  • Block explorer information (Zentrace)
  • Marked as a testnet.

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🦋 Changeset detected

Latest commit: 3342457

The changes in this PR will be included in the next version bump.

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