Fix cursor positioning for multiline chat input query (microsoft#269067)

* Initial plan

* Fix cursor positioning for multiline chat input

Co-authored-by: benibenj <44439583+benibenj@users.noreply.github.com>

* Use getLastPosition helper for consistency

Co-authored-by: benibenj <44439583+benibenj@users.noreply.github.com>

* Add null check for model as suggested by @benibenj

Co-authored-by: benibenj <44439583+benibenj@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: benibenj <44439583+benibenj@users.noreply.github.com>

Author: Copilot

src/vs/workbench/contrib/chat/browser/chatInputPart.ts

@@ -985,7 +985,10 @@ export class ChatInputPart extends Disposable implements IHistoryNavigationWidge
 	setValue(value: string, transient: boolean): void {
 		this.inputEditor.setValue(value);
 		// always leave cursor at the end
-		this.inputEditor.setPosition({ lineNumber: 1, column: value.length + 1 });
+		const model = this.inputEditor.getModel();
+		if (model) {
+			this.inputEditor.setPosition(getLastPosition(model));
+		}
 
 		if (!transient) {
 			this.saveCurrentValue(this.getInputState());