feat(solana-wallet-snap): route asset reads through Core when migration is on

[ulissesferreira]

Explanation

Part of migrating the Solana assets-controller staged migration into this monorepo (PR 4 of 6). Follows the Tron WPN-1497 routing approach from #145: a thin if in AssetsService rather than inlining AssetsProvider calls.

Solana is simpler than Tron: there are no snap-owned assets, so the flag-on path does not fetch, persist, or publish leftover Snap balances, and does not monitor addresses for snap-owned changes.

• Pass RemoteFeatureFlagsProvider into AssetsService and resolve the Solana assets migration flag via #shouldReturnAssetsFromCore / isUsingCoreAssets
• Route getAccountAssetByID, getAccountAssetsByIDs, getAccountAssetsByScope, and getAccountAssets through CoreAssetsAdapter when migration is active
• When migration is on, fetch returns [] and saveMany is a no-op (Core already tracks fungibles)
• KeyringAccountMonitor still discovers receive transactions over websockets, but skips balance persistence when the flag is on
• When off, reads/fetch/save still go through SnapAssetsAdapter
• AssetsService stays a thin facade

References

• Jira: WPN-1476 / WPN-1652
• Stacks on: feat(solana-wallet-snap): add read-only CoreAssetsAdapter and mapControllerAsset #122
• Tron parallel: feat(tron-wallet-snap): route asset reads through Core when migration is on #145
• Next: hardcode Core reads and remove migration flags (#124)

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