Explanation

The AssetsController needs to take over assets ingestion (balances, metadata,
prices) for the non-EVM networks currently owned by their Snaps — Solana,
Stellar, and Tron — but this has to happen gradually and be reversible per
network. Previously the AssetsController/AccountsApiDataSource had no notion
of a per-network migration state: the Accounts API data source hardcoded an
eip155-only filter for the chains it surfaced as active, so there was no safe
way to stage the rollout or roll it back without a code change.
This PR adds stage-gated ingestion driven by the
RemoteFeatureFlagController, so the migration for each network can be advanced
and rolled back independently via LaunchDarkly:

• A new snaps-assets-migration util module defines the per-network rollout
  model: the SnapsAssetsMigrationStage enum (Off → ReadAssetsControllerWithFallback
  → ReadAssetsControllerWithoutFallback → ReadAssetsControllerOnly), the
  per-network flag keys (networkAssetsSnapsMigrationSolana,
  networkAssetsSnapsMigrationStellar, networkAssetsSnapsMigrationTron), and
  helpers to resolve and evaluate a stage from remote feature flag state
  (parseSnapsAssetsMigrationStage, isMigrationStageActive,
  getSnapsAssetsMigrationNamespace, getMigrationStages, shouldSupportChain).
• The AssetsController resolves each network's stage from
  RemoteFeatureFlagController state (read via the
  RemoteFeatureFlagController:getState messenger action). Migration networks
  are ingested (Account Activity WebSocket + Accounts API) only from
  ReadAssetsControllerWithFallback onward and left to the Snap when the stage
  is Off — which is also the fail-safe when the flag is missing or the
  controller is unavailable. Non-migration namespaces (e.g. eip155) are never
  gated.
• The AccountsApiDataSource now gates the networks it surfaces as active
  chains on the same per-network stage (replacing the hardcoded eip155-only
  filter). It also subscribes to RemoteFeatureFlagController:stateChange
  (mirroring core-backend's AccountActivityService) so newly-enabled
  networks are picked up — and disabled ones dropped — without waiting for the
  periodic refresh. A selector reduces the flags to a primitive migration-stage
  signature so the handler only fires when a stage actually changes.

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