feat(snap-account-service): add getLegacySnapKeyring

[ccharly]

Explanation

Add a getLegacySnapKeyring to replace all use of getSnapKeyring in each clients.

References

N/A

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 that can create a legacy Snap keyring via KeyringController:withController, which touches keyring management and could impact account/keyring state if misused. Scope is otherwise contained to the snap-account-service package with dedicated tests.

Overview
Adds a new SnapAccountService:getLegacySnapKeyring messenger action that atomically returns the existing legacy Snap keyring (KeyringTypes.snap) or creates it via KeyringController:withController/addNewKeyring, with logging when creation occurs.

Updates action type unions and tests to cover existing-keyring, create-if-missing, and error propagation behavior, and adds @metamask/eth-snap-keyring (plus @metamask/keyring-utils for typing) dependencies alongside changelog entries.

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