[Split #88320] PR 5 - Extract useReportActionsScroll

[rlinoz]

Goal: Pull scroll state, Pusher new-action handling, and scroll imperatives into useReportActionsScroll.

Scope:

• Add src/hooks/useReportActionsScroll.ts
• Update ReportActionsList.tsx into a thinner consumer
• Add unit coverage for Pusher handling, scroll imperatives, and IOU-error effect

Risk: Medium. Scroll anchoring and incoming messages are sensitive.

Depends on: #89765

Parent issue: #88320
Source plan: #88320 (comment)

Issue Owner

Current Issue Owner: @DylanDylann