[Agent Builder] - Disable "Open in full screen" before conversation persistence (elastic#267699)

## Summary

Fixes a bug where navigating to the full-screen chat from the embedded
flyout immediately after starting a new conversation opens a new empty
session instead of the current one. This occurred because the
`conversationId` was not yet persisted by the server.

This PR addresses the issue by disabling the "Open Full Screen" menu
item until a stable `conversationId` is available.

### What was changed:
* **UI/UX:** Disabled the `AgentBuilderFullScreenMenuItem` while the
conversation is being created (no stable `conversationId` yet).
* **Tooltip:** Wrapped the disabled `EuiContextMenuItem` in an
`EuiToolTip` (using a wrapper `span` to bypass EUI pointer-event
limitations on disabled items) to explain to users why the action is
temporarily unavailable.
* **Guard clause:** Updated `handleOpenFullScreen` in
`more_actions_button.tsx` to early return if `conversationId` is
missing, preventing inconsistent routing.

### How to test:
**Manual testing steps:**
1. Open the Agent Builder embedded flyout.
2. Start a new conversation and immediately open the "More Actions"
(...) menu.
3. Observe that the "Open Full Screen" button is **disabled** and shows
a tooltip when hovered.
4. Wait for the server response (until the `conversationId` is
persisted).
5. Observe that the button becomes **enabled**.
6. Click "Open Full Screen" and confirm it successfully navigates to the
current conversation context (URL should contain the correct
`conversationId`).

Author: ashatova

x-pack/platform/plugins/shared/agent_builder/public/application/components/conversations/conversation_header/more_actions_button.tsx

@@ -11,6 +11,7 @@ import {
   EuiPopover,
   EuiContextMenuPanel,
   EuiSpacer,
+  EuiToolTip,
 } from '@elastic/eui';
 import { i18n } from '@kbn/i18n';
 import React, { useCallback, useMemo, useState } from 'react';
@@ -54,6 +55,12 @@ const fullscreenLabels = {
   fullScreen: i18n.translate('xpack.agentBuilder.conversationActions.fullScreen', {
     defaultMessage: 'Open in full screen',
   }),
+  fullScreenDisabledTooltip: i18n.translate(
+    'xpack.agentBuilder.conversationActions.fullScreenDisabledTooltip',
+    {
+      defaultMessage: 'Full-screen mode is available once this conversation has been created.',
+    }
+  ),
   addToDataset: i18n.translate('xpack.agentBuilder.conversationActions.addToDataset', {
     defaultMessage: 'Add conversation to dataset',
   }),
@@ -147,6 +154,7 @@ export const MoreActionsButton: React.FC<MoreActionsButtonProps> = ({ onCloseSid
 
   const handleOpenFullScreen = useCallback(() => {
     if (!application) return;
+    if (!conversationId) return;
 
     setIsPopoverOpen(false);
     onCloseSidebar?.();
@@ -158,6 +166,17 @@ export const MoreActionsButton: React.FC<MoreActionsButtonProps> = ({ onCloseSid
     navigateToAgentBuilderUrl(path);
   }, [application, agentId, conversationId, navigateToAgentBuilderUrl, onCloseSidebar]);
 
+  const fullScreenMenuItemLabel = useMemo(() => {
+    if (conversationId) {
+      return fullscreenLabels.fullScreen;
+    }
+    return (
+      <EuiToolTip content={fullscreenLabels.fullScreenDisabledTooltip}>
+        <span tabIndex={0}>{fullscreenLabels.fullScreen}</span>
+      </EuiToolTip>
+    );
+  }, [conversationId]);
+
   const addToDatasetMenuItem = showAddToDatasetItem
     ? [
         <EuiContextMenuItem
@@ -189,10 +208,11 @@ export const MoreActionsButton: React.FC<MoreActionsButtonProps> = ({ onCloseSid
             key="full-screen"
             icon="fullScreen"
             size="s"
+            disabled={!conversationId}
             data-test-subj="agentBuilderFullScreenMenuItem"
             onClick={handleOpenFullScreen}
           >
-            {fullscreenLabels.fullScreen}
+            {fullScreenMenuItemLabel}
           </EuiContextMenuItem>,
         ]
       : []),