Assistant: Improves inline chat text editing (#10911)

Fixes an issue in Assistant's streaming text replacement logic when
inserting text across multiple lines in inline chat. The existing logic
did not reset the insertion position's character location when moving to
a new line, leading to text being inserted mid-way through a line -
possibly interposing with existing text.

Addresses #10914

Author: samclark2015

extensions/positron-assistant/src/replaceSelectionProcessor.ts

@@ -102,7 +102,12 @@ export class ReplaceSelectionProcessor {
 		const lines = text.split(/\r?\n/);
 		const lineDelta = lines.length - 1;
 		const characterDelta = lines.at(-1)!.length;
-		this._insertPosition = this._insertPosition!.translate(lineDelta, characterDelta);
+		if (lineDelta === 0) {
+			this._insertPosition = this._insertPosition!.translate(lineDelta, characterDelta);
+		} else {
+			// If we added new lines, reset the character position to the length of the last line.
+			this._insertPosition = this._insertPosition.translate({ lineDelta }).with({ character: characterDelta });
+		}
 	}
 
 	private onReplaceSelectionClose(): void {