Clarify custom-chain bundler requirements

[defiQUG]

Summary

• clarify in the README that chain overrides resolve custom EVM chains for standard Engine operations, but do not by themselves enable hosted bundler/paymaster support
• expand the generic invalid-chain error message so operators using smart wallet / UserOp routes are not pointed only at chain overrides

Why

Engine supports custom chain overrides, but the UserOp routes still post to https://{chainId}.bundler.thirdweb.com. In practice that means a custom chain can be valid for normal Engine reads/writes/deployments while still failing for smart wallet flows if hosted bundler support is unavailable.

This change keeps the scope narrow and just makes that boundary explicit.

Validation

• local diff review only
• attempted to run the repo build/lint flow via Corepack/Yarn in this environment, but dependency install did not complete cleanly enough to provide working tsc / biome binaries before publish

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PR-Codex overview

This PR enhances error messaging and documentation regarding custom chains in the project. It clarifies the requirements for using smart wallets and UserOp routes with custom chains.

Detailed summary

• Updated the error message in the badChainError function to include information about bundler support requirements for smart wallet and UserOp routes.
• Added a new section in README.md about custom chains, explaining chain overrides and their dependencies on bundler support.

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