fix(assets-controller): seed native balances after account refresh

[salimtb]

Zero-balancing for new accounts ran before getAssets on account-tree changes, so assetsBalance was often never populated for the new account. Seed native zero balances using the refreshed account list after the fetch.

• Fetch balances via RpcDataSource when a user switches EVM accounts, enables RPC-only networks, switches account groups, or adds a new account to the account tree
• Fixes missing assetsBalance for newly added accounts on custom/RPC networks (e.g. DXC / eip155:50) where assetsInfo was present but balances were not fetched

Explanation

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

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Note

Medium Risk
Changes balance refresh timing in AssetsController for account-tree and subscription flows; localized but affects wallet balance display on custom/RPC networks.

Overview
Fixes cases where assetsBalance stayed empty for new or RPC-only-network accounts even when metadata was present, by tightening when AssetsController force-refreshes balances.

On account tree updates, the controller now tracks newly added accounts and, after the usual group-wide getAssets and re-subscribe, runs a second forced fetch scoped to those new accounts so the slow/RPC pipeline can populate balances (e.g. custom chains like eip155:50).

Documents the same class of fix for account group switches and newly enabled RPC-only networks in the changelog. #handleEnabledNetworksChanged only refactors to reuse a local accounts variable when fetching added chains—behavior unchanged.

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