Fix button missing rounded corners when hasSingleAction is true

Add CSS rule for button.split-left:only-child to restore full
border-radius and border-right when the split-left button is the
only child in the container (i.e., when split/split-right elements
are not rendered because hasSingleAction is true).

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>
Agent-Logs-Url: https://github.com/microsoft/vscode-pull-request-github/sessions/bf4729e0-f0d3-4c05-88a6-cf04d33863be

Author: Copilot

webviews/common/common.css

@@ -430,6 +430,11 @@ button.split-left {
 	border-right: none;
 }
 
+button.split-left:only-child {
+	border-radius: 4px;
+	border-right: 1px solid var(--vscode-button-border, transparent);
+}
+
 .split {
 	background-color: var(--vscode-button-background);
 	border-top: 1px solid var(--vscode-button-border);