feat: Expose KeyringController:exportSeedPhrase through messenger

[GuillaumeRx]

Explanation

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

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
Adds a new messenger-exposed method for exporting the HD seed phrase, which is security-sensitive despite being mostly wiring/type-surface changes.

Overview
Exposes seed phrase export via the KeyringController messenger. This adds exportSeedPhrase to MESSENGER_EXPOSED_METHODS, introduces the KeyringController:exportSeedPhrase method action type, includes it in the KeyringControllerMethodActions union, and re-exports the new action type from src/index.ts.

Updates the keyring-controller changelog to announce the new messenger method.

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