feat(snap-account-service): add migration logic + keyring v2 support

[ccharly]

Explanation

Add migration logic to migrate from the legacy Snap keyring to the new architecture with 1 Snap keyring v2 per Snaps.

Also enhance :ensureReady so consumers can safely call :withKeyringV2 to get the associated Snap keyring v2 instance before using it.

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

High Risk
High risk because it introduces a one-time migration that moves Snap-managed accounts between keyring implementations and changes how keyring messages/selected-account forwarding are routed, which can impact account availability and state integrity.

Overview
Adds a concurrent-safe SnapAccountService.migrate() and updates ensureReady() to always migrate legacy Snap keyring accounts into per-Snap v2 keyrings and auto-create a missing v2 keyring for a Snap before proceeding.

Switches keyring message handling and selected-account forwarding to target v2 Snap keyrings (KeyringController:withKeyringV2/withKeyringV2Unsafe), including special-casing GetSelectedAccounts, and updates multichain-account-service Snap providers to use withKeyringV2 plus v2 Snap keyring APIs (createAccount(s) signature and deleteAccount instead of removeAccount).

Updates tests/changelogs accordingly and adds @metamask/keyring-api + @metamask/keyring-snap-sdk dependencies for the new v2/event-based integration.

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