feat!: Reduce cronjob write churn by persisting dates separately

[MajorLift]

Summary

• CronjobController rescheduling is its most frequent write by a wide margin — a Snap on a PT30S schedule reschedules every thirty seconds — and it changes exactly one field, events[id].date. It currently goes through CronjobControllerStateManager.set, which hands the client the whole event map to re-serialise on every tick.
• This adds setEventDate(id, date) to the state manager interface and routes #schedule through it. The controller's in-memory state is updated the same way as before; only the persistence call changes.
• Clients can now store dates apart from the rest of the state and merge them back in getInitialState. Nothing in this PR requires them to — a client that keeps writing the whole blob from setEventDate behaves exactly as it does today.
• The three remaining set calls — add, post-execution cleanup, cancel — are structural changes to the event map and are left alone. They fire rarely.

BREAKING: CronjobControllerStateManager is exported, so every implementer must add setEventDate. The minimal migration keeps existing behaviour:

setEventDate(id, date) {
  this.set({ ...this.#state, events: { ...this.#state.events, [id]: { ...this.#state.events[id], date } } });
}

Follows on from MetaMask/snaps#3539 (use a custom state manager for the cronjob controller), which introduced this interface as a way to avoid persisting all client state on every cronjob write. This narrows the remaining case that interface did not cover.

Client-side follow-up, tracked separately: the extension's CronjobControllerStorageManager writes to browser.storage.local under temp-cronjob-storage. Backing setEventDate with StorageService there is the point of this change, and is one of the writes catalogued in MetaMask/metamask-extension#44802 (flag controller-state writes that bypass PersistenceManager), under MetaMask/metamask-extension#44253 (Storage Resilience E4: zero controller writes while idle). That work is blocked on this landing and being released.

Known limitation

If a client stores dates where they can be lost independently of the rest of the state, a missing date makes DateTime.fromISO(undefined) yield NaN. #startTimer then computes NaN milliseconds; both its guards (ms > DAILY_TIMEOUT and ms <= 0) are false against NaN, so it reaches new Timer(NaN), whose constructor asserts !Number.isNaN(ms) and throws TypeError: Can't start a timer with NaN time.

The blast radius is wider than the affected event:

• init() throws, so the client sees an initialization failure.
• #reschedule iterates events in a plain for loop, so every event ordered after the bad one is never scheduled either.
• The daily timer is armed before the throw, but its callback is this.#reschedule(); this.#start(); — the throw skips the re-arm, so background scheduling stops for the rest of the session at the first daily tick.

This is unreachable today: date is always written in the same operation as the rest of the event, so it cannot go missing on its own. It becomes reachable only once a client stores dates separately, which is what this PR enables. Until recovery handling lands, a client adopting setEventDate should keep dates in a store that fails together with the main state.

Recovering instead of throwing is tractable — schedule and scheduledAt are both immutable and stay in the main state, so a lost date is reconstructible in every case — and is tracked as a follow-up rather than included here.

Test plan

• yarn workspace @metamask/snaps-controllers jest src/cronjob/CronjobController.test.ts — 27 passed, 4 snapshots passed. The 26 pre-existing tests pass unchanged, so the refactor is behaviour-preserving.
• New test persists a reschedule through 'setEventDate', without rewriting all state asserts setEventDate is called with the event ID and set is not called at all during a reschedule.
• Negative control: reverting #schedule to this.#stateManager.set(nextState) fails that test (Number of calls: 0), and restoring the change passes it — so the assertion is load-bearing rather than vacuous.
• The mock state manager in the test file now stores dates in a separate Map and merges them in getInitialState, modelling the split a real client implements. Every existing test exercises that merge path.
• eslint clean on both changed files. tsc -b produces no errors in snaps-controllers (17 pre-existing failures in snaps-utils, unrelated).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (ff9ec9c) to head (3d50194).

Additional details and impacted files

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##             main    #4107   +/-   ##
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  Coverage   98.59%   98.59%           
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  Files         429      429           
  Lines       12495    12495           
  Branches     1976     1976           
=======================================
  Hits        12320    12320           
  Misses        175      175           

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