chore: Expose KeyringController:signTransaction through messenger

[Mrtenz]

Explanation

This adds KeyringController:signTransaction to MESSENGER_EXPOSED_METHODS and exports the type, as this was missed in #8391.

References

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

Medium Risk
Expands the controller’s messenger API to allow remote callers to request transaction signatures, which is security-sensitive despite being a small change. Risk is mainly around new call surface/permissions and ensuring consumers handle the new action correctly.

Overview
Enables KeyringController:signTransaction to be invoked through the KeyringController messenger by adding it to the exposed method allowlist.

Adds and exports the corresponding KeyringControllerSignTransactionAction type (and updates the method-action union), and records the new messenger exposure in the package changelog.

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