fix(chat): preserve scroll anchor when user toggles collapsible thinking/subagent sections (#274731)

[maruthang]

What — Manually expanding or collapsing a thinking/subagent/hook/markdown section in the chat list no longer snaps the viewport to the bottom. Previously-visible content stays visually put; the expand grows downward into unused space.

Why — ChatListWidget._updateElementHeight auto-scrolls the list to the bottom whenever a row grows and the user was already pinned to the bottom. That's correct during streaming (content growing from below must stay visible) but wrong when the user clicks to expand a collapsed section: the expanded content appears above the anchor, so every added pixel shifts everything else upward (#274731).

How — Distinguish user-driven from programmatic height changes. ChatCollapsibleContentPart fires a bubbling CustomEvent('chatCollapsibleUserToggle') from its click handler; ChatListWidget records the timestamp on receive and skips _withPersistedAutoScroll within a 250 ms window. Programmatic setExpanded (used during streaming) doesn't dispatch the event, so auto-scroll during streaming is unchanged.

Test plan — Manual (from the issue): agent-mode prompt "Look at the git changes in the last 24 hours…", wait for streaming, scroll to bottom, click to expand the thinking section. Pre-fix: viewport jumps upward. Post-fix: content above stays put. Also verified streaming auto-scroll still works. Added 2 unit tests in chatThinkingContentPart.test.ts asserting the click dispatches exactly one bubbling event.

Applies to every ChatCollapsibleContentPart subclass, not just thinking.

Fixes #274731