feat(account-tree-controller): add get/set account group methods

[hmalik88]

Explanation

Added getAccountFromSelectedAccountGroup and setSelectedAccountGroupByAccountId methods

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
Changes how the selected account is synced to AccountsController and introduces APIs that drive account/group selection; behavior is well-tested but affects core wallet selection paths.

Overview
Adds getAccountFromSelectedAccountGroup and setSelectedAccountGroupByAccountId on AccountTreeController, exposed via messenger actions and package exports, with changelog and broad unit tests.

getAccountFromSelectedAccountGroup is intended as the group-based replacement for AccountsController:getSelectedAccount / getSelectedMultichainAccount. Without a chain ID it applies the existing EVM-priority pick (first EVM in the group, else first account). With an optional CAIP-2 chainId it returns the first account in the selected group whose scopes match, validates the chain ID, and returns undefined when nothing matches.

setSelectedAccountGroupByAccountId resolves the account’s group from the internal tree map and delegates to setSelectedAccountGroup, throwing if the account is not in the tree.

Internal selection logic is refactored: #getDefaultAccountFromAccountGroupId becomes #getAccountFromAccountGroupId, returning a full InternalAccount instead of an ID. #setSelectedAccountGroup now forwards accountToSelect.id to AccountsController:setSelectedAccount (previously passed the account object).

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