fix(ui5-flexible-column-layout): disable resizing for both separators (#11411)

Problem:
When disableResizing="true" was set, only the start-mid column separator was
disabled, while the mid-end separator remained interactive and resizable.

Solution:
Updated the onSeparatorPress method to check both separator types and prevent
interaction for the end separator when disableResizing is true
Modified the endSeparatorTabIndex getter to make it consistent with the start
separator by removing the explicit -1 return value, which makes it
unfocusable when disableResizing is true

Fixes: #11402

Author: NakataCode

packages/fiori/src/FlexibleColumnLayout.ts

@@ -486,7 +486,8 @@ class FlexibleColumnLayout extends UI5Element {
 			return;
 		}
 		const pressedSeparator = (e.target as HTMLElement).closest(".ui5-fcl-separator") as HTMLElement;
-		if (pressedSeparator.classList.contains("ui5-fcl-separator-start") && !this.showStartSeparatorGrip) {
+		if ((pressedSeparator.classList.contains("ui5-fcl-separator-start") && !this.showStartSeparatorGrip)
+			|| (pressedSeparator.classList.contains("ui5-fcl-separator-end") && !this.showEndSeparatorGrip)) {
 			return;
 		}
 
@@ -1091,7 +1092,6 @@ class FlexibleColumnLayout extends UI5Element {
 		if (this.showEndSeparatorGrip) {
 			return 0;
 		}
-		return -1;
 	}
 
 	get media() {