chore: Expose missing MultichainAccountService methods through the messenger

[GuillaumeRx]

Explanation

This exposes missing methods used in the clients through the messenger after #7976

References

Progresses https://consensyssoftware.atlassian.net/browse/WPC-989

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
Low risk: this only exposes an existing init method through the messenger and updates exported types/changelog, without changing initialization behavior or data handling.

Overview
Exposes MultichainAccountService:init through the service messenger by adding it to MESSENGER_EXPOSED_METHODS and introducing the corresponding MultichainAccountServiceInitAction type.

Updates public exports (src/index.ts) and the package changelog to reflect the newly available messenger action and action type.

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