[State Sync Redesign 2.3] Simplify `getState()` to iterate `controllersByName`

`getState()` currently calls `memStore.getFlatState()`. Replace with direct iteration over `controllersByName`, merging `controller.state` for each. Also update Sentry's `getSentryAppState` in `background.js`.

## Steps

1. Implement `getState()` as `Object.fromEntries(controllersByName.entries().map(...))`
2. Decide: return flat (for backward compat during migration) or controller-keyed
3. Update `setupSentryGetStateGlobal` in `background.js` to use new `getState()`

## Additional consumers identified (PoC)

The PoC branch (`jongsun/poc/sync-state-redesign`) identified additional flat-state consumers that depend on `getState()` returning flat state:

- **`_getMetaMaskState()`**: Used by `getIsSmartTransaction` and `selectAllEnabledNetworkClientIds` selectors. These selectors expect `{ metamask: flatState }` shape.
- **`createEip1193MethodMiddleware`**: Receives `metamaskState: this.getState()` as a one-time snapshot. See #40182.
- **`controller-init` `getFlatState`**: `transaction-controller-init.ts` passes `getFlatState()` to smart transaction helpers. See #40181.
- **`createCancelTransaction` / `createSpeedUpTransaction`**: Return `this.getState()` to UI callers. With per-controller subscriptions, these no longer need to return state.
- **`_trackTransactionFailure`**: Reads `metamaskState.accounts` — can be replaced with direct `accountTrackerController.state.accounts`.

## Sub-issues

- #40181 — Remove flat state from `controller-init` smart transaction helpers
- #40182 — Remove flat state from `createEip1193MethodMiddleware`

## Files

- `app/scripts/metamask-controller.js`
- `app/scripts/background.js`

## Risk

Low — lazy read, not subscription-based.

## Dependencies

None

## Context

Part of [State Sync Redesign — Epic 2: Background cleanup].

Start in app/scripts/metamask-controller.js with getState(), controllersByName, and the listed flat-state consumers; then inspect setupSentryGetStateGlobal in app/scripts/background.js. Review sub-issues #40181 and #40182 before choosing the migrated state shape. Done means getState and Sentry use the agreed shape without breaking the identified consumers.

Written by the indexing model from the issue text.