Fix TreeError in AgentSessionsView

[benibenj]

Copilot Generated Description: Catch potential errors when revealing a session in the Agent Sessions view to handle cases where the tree model may be temporarily inconsistent during async refresh. This change prevents unhandled errors from occurring when attempting to access the relative position of a session.
Fixes #309891

[Copilot]

Pull request overview

This PR hardens the Agent Sessions view’s “reveal session” behavior against transient tree model inconsistencies that can occur during async refresh, preventing unhandled tree errors when trying to scroll a session into view (#309891).

Changes:

• Wraps the tree visibility check (getRelativeTop) and reveal call in a try/catch to avoid propagating tree-model exceptions.
• Returns false if the tree operation fails during refresh, instead of throwing.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts	Adds error handling around tree reveal logic to avoid unhandled errors during async refresh.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts:850

• setFocus/setSelection can also throw TreeError if the tree model changes between hasNode(session) and these calls (they resolve nodes/list indexes internally). To fully prevent the unhandled TreeError, consider moving these calls inside the same try/catch (or guarding similarly) so transient refresh races don't still crash.

		this.sessionsList.setFocus([session]);
		this.sessionsList.setSelection([session]);

• Files reviewed: 1/1 changed files
• Comments generated: 1