Closes #22759

Summary

buildEventsState inserted events into the id lookups without validating the resolved id, so duplicate or missing ids silently corrupted the scheduler state with no feedback:

• Duplicate ids: the second event overwrote the first in eventModelLookup and processedEventLookup, but eventIdList kept both copies. The array and the lookups disagreed, which surfaced as duplicate React keys (occurrences are keyed by String(event.id)) and events rendering with another event's data.
• Missing ids: when the resolved id was undefined, every id-less event collided on the same key, so only one survived. Silent data loss.

The premium lazy-loading cache had the same gap: its default getId returns event.id, and upsert keyed entries by String(this.getId(event)), so id-less events all collapsed onto the literal "undefined" key and could be wrongly evicted as server-side deletes.

SchedulerEvent.id is typed as required, so TypeScript users are nudged at compile time. The real victims are JavaScript users, dynamically sourced data, and the dataSource path where ids resolve at runtime.

Changes

• Add checkSchedulerEventIdIsValid(id, event) in SchedulerStore.utils.ts, mirroring the Data Grid's checkGridRowIdIsValid. It throws a clear MUI X Scheduler error (what happened, why it is a problem, how to fix it) when the id is null / undefined, including the offending event and pointing to eventModelStructure.idGetter as the per-event id source, or remains a valid id.
• Use the helper in buildEventsState. On a missing id it throws. On a duplicate id it emits a dev-only warnOnce and skips re-pushing the id, so eventIdList stays consistent with the last-wins lookups. This removes the list/lookup mismatch and the duplicate React keys in production, not just in development.
• Reuse the same helper in the premium cache's upsert, validating the resolved id before stringifying it. Because setRange funnels every event through upsert, this also fixes the wrong eviction of id-less events.

This keeps last-wins semantics for duplicates, matching the Data Grid's lenient direction, while fixing the scheduler-specific corruption that was worse than the Grid's (the Grid never diverged its id list from its lookup; the scheduler did).

Behavior

• Missing id: throws MUI X Scheduler: All events must have a unique \id`...` (registered as error code 287).
• Duplicate id: warns once in development, keeps the last event for that id, and renders it a single time.

Testing

• event.SchedulerStore.test.ts: a missing id throws; two events sharing an id warn in dev, dedupe the id list, and keep the last event (verified across EventCalendarStore, EventCalendarPremiumStore, and EventTimelinePremiumStore).
• dataSource.test.ts: upsert and setRange throw on a missing id instead of colliding on "undefined".
• Ran pnpm extract-error-codes (added code 287), typecheck, eslint, and prettier.
• Full suites for x-scheduler-internals and x-scheduler-internals-premium pass (1235 passed, 3 skipped).

How to reproduce

1. Render a scheduler with two events without an id (or a dataSource whose getter cannot resolve one). Before this change one event silently disappears; after it throws a clear error.
2. Render two events with the same id and different titles. Before this change the event renders twice with duplicate React keys and mismatched data; after it renders once with a dev warning.

• I have followed (at least) the PR section of the contributing guide.