fix: prevent crash on task assignee change (#85974)

[arlo-owls]

Fixed Issues

$250 #85974

Tests

1. Create a task with an existing user as assignee
2. Open the task → assignee
3. Select another existing user from contacts
4. Verify app does NOT crash
5. Verify assignee is updated correctly

Root Cause

PR #85539 moved participants: {[assigneeAccountID]: null} inside the isOptimisticPersonalDetail() conditional in getTaskAssigneeChatOnyxData. For existing users, isOptimisticPersonalDetail returns false, so the participant cleanup never runs on success. This leaves stale/duplicate participant data that crashes the app on Android when re-rendering.

Fix

Move the participant cleanup back outside the isOptimisticPersonalDetail conditional so it always runs for optimistic chat reports, while keeping the personal details cleanup inside the conditional (since that only applies to optimistic users). This restores the pre-#85539 behavior for participant cleanup.

[github-actions]

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