.NET: HarnessConsole: Improve rendering perf / reduce flickering

[westey-m]

Motivation and Context

Description

• Reduce the amount of re-rendering for the main scroll area

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR updates the .NET Harness console rendering path to reduce unnecessary re-renders (and resulting flicker), primarily by changing how the scroll area is updated during streaming output.

Changes:

• Simplifies HarnessAppComponent layout by removing the separate “last item” text panel and rendering all scroll content via TextScrollPanel.
• Refactors TextScrollPanel to treat the last item as dynamic (re-rendered each update) while keeping earlier items “finalized”.
• Reduces line-clearing work in ListSelection by erasing only the remainder of each line instead of the entire line.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
dotnet/samples/02-agents/Harness/Harness_Shared_Console/HarnessAppComponent.cs	Removes dedicated last-item rendering and relies on TextScrollPanel for the full scroll region.
dotnet/samples/02-agents/Harness/ConsoleReactiveFramework/ConsoleReactiveMemo.cs	Adds a small memoization helper for caching mapped outputs when inputs are unchanged.
dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/TextScrollPanel.cs	Implements “dynamic last item” rendering to minimize scroll-area redraws.
dotnet/samples/02-agents/Harness/ConsoleReactiveComponents/ListSelection.cs	Switches from erasing entire lines to erasing to end-of-line to reduce redraw work.