fix(controller-utils): update Sei Mainnet explorer to seiscan.io

[jeremy-consensys]

Explanation

The current default block explorer for Sei Mainnet (seitrace.com) is being
decommissioned. This PR updates the BlockExplorerUrl entry for
BuiltInNetworkName.SeiMainnet to its replacement, seiscan.io.

seiscan.io is an Etherscan-family explorer. This is a UI-URL change only —
@metamask/controller-utils exposes the user-facing block-explorer URL (the
one consumers concatenate /tx/<hash> or /address/<addr> onto), and the
hostname swap is the only thing that needs to land here.

Clients (metamask-extension, metamask-mobile) will pick this up via a
@metamask/controller-utils bump and ship a state migration to rewrite
existing users' stored Sei explorer URL.

References

• Related PR in MetaMask/metafi-sdk for the same explorer migration in @codefi/metafi-common.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Single constant URL change plus changelog update; no behavior or data-handling logic changes beyond where links point.

Overview
Updates @metamask/controller-utils to use https://seiscan.io as the default BlockExplorerUrl for BuiltInNetworkName.SeiMainnet, replacing the deprecated https://seitrace.com.

Documents the change in the package changelog under Unreleased.

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