…l position (#13744)

### What this PR does

Before this PR:

Agent chat page created a separate `useRef` for `scrollContainerRef`
instead of using the one provided by `useScrollPosition`, causing the
hook to never observe the actual scroll container. As a result, scroll
positions were not memorized when switching between agent sessions.

After this PR:

Uses `containerRef` returned from `useScrollPosition` as the scroll
container ref, so the hook correctly tracks and restores scroll
positions for agent sessions.

### Why we need it and why it was done in this way

The `useScrollPosition` hook needs its own `containerRef` to be attached
to the scrollable DOM element in order to read/write `scrollTop`. The
previous code ignored this ref and created a standalone one, breaking
the memorization mechanism.

The following tradeoffs were made: N/A

The following alternatives were considered: N/A

### Breaking changes

None

### Special notes for your reviewer

Minimal one-line fix — just replaced the standalone `useRef` with the
`containerRef` from the hook.

### Checklist

- [x] PR: The PR description is expressive enough and will help future
contributors
- [x] Code: [Write code that humans can
understand](https://en.wikiquote.org/wiki/Martin_Fowler#code-for-humans)
and [Keep it simple](https://en.wikipedia.org/wiki/KISS_principle)
- [x] Refactor: You have [left the code cleaner than you found it (Boy
Scout
Rule)](https://learning.oreilly.com/library/view/97-things-every/9780596809515/ch08.html)
- [ ] Upgrade: Impact of this change on upgrade flows was considered and
addressed if required
- [ ] Documentation: A [user-guide update](https://docs.cherry-ai.com)
was considered and is present (link) or not required. Check this only
when the PR introduces or changes a user-facing feature or behavior.
- [x] Self-review: I have reviewed my own code (e.g., via
[`/gh-pr-review`](/.claude/skills/gh-pr-review/SKILL.md), `gh pr diff`,
or GitHub UI) before requesting review from others

### Release note

```release-note
fix: Fixed agent chat page not memorizing scroll position when switching between sessions
```