[Discover Tabs] Tab preview improvements (elastic#235176)

- Closes elastic#231426

## Summary

This PR:
- decreases tab label text size inside the tab preview
- removes the browser title for the tab
- changes the length limit to 120
- and allows to select text inside the tab preview popover (previously
the popover was getting closed too soon)

<img width="396" height="274" alt="Screenshot 2025-09-16 at 11 19 41"
src="https://github.com/user-attachments/assets/866f5dec-5ae6-462e-b539-bc883a1c0ace"
/>



### Checklist

- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [x] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

Author: jughosta

src/platform/packages/shared/kbn-unified-tabs/src/components/tab/edit_tab_label.tsx

@@ -41,7 +41,7 @@ export const EditTabLabel: React.FC<EditTabLabelProps> = ({ item, onLabelEdited,
 
   const onSubmit = useCallback(
     async (newLabel: string) => {
-      if (!newLabel) {
+      if (!newLabel || newLabel.length > MAX_TAB_LABEL_LENGTH) {
         return;
       }
 
@@ -93,7 +93,9 @@ export const EditTabLabel: React.FC<EditTabLabelProps> = ({ item, onLabelEdited,
       value={value}
       maxLength={MAX_TAB_LABEL_LENGTH}
       isLoading={submitState === SubmitState.submitting}
-      isInvalid={submitState === SubmitState.error || !value.trim()}
+      isInvalid={
+        submitState === SubmitState.error || !value.trim() || value.length > MAX_TAB_LABEL_LENGTH
+      }
       onChange={onChange}
       onKeyDown={onKeyDown}
       onBlur={submitState !== SubmitState.submitting ? onExit : undefined}

src/platform/packages/shared/kbn-unified-tabs/src/components/tab/tab.tsx

@@ -210,6 +210,7 @@ export const Tab: React.FC<TabProps> = (props) => {
                   // Truncation width must be equal to max tab width minus padding
                   width={tabsSizeConfig.regularTabMaxWidth - euiTheme.base}
                   truncation="middle"
+                  title=""
                 />
               </EuiText>
             </div>

src/platform/packages/shared/kbn-unified-tabs/src/components/tab_preview/tab_preview.tsx

@@ -140,8 +140,8 @@ export const TabPreview: React.FC<TabPreviewProps> = ({
   }, [previewTimer, setShowPreview]);
 
   return (
-    <div>
-      <span onMouseEnter={handleMouseEnter} onMouseLeave={handleMouseLeave} ref={tabRef}>
+    <div onMouseLeave={handleMouseLeave}>
+      <span onMouseEnter={handleMouseEnter} ref={tabRef}>
         {children}
       </span>
       <EuiPortal>
@@ -166,6 +166,7 @@ export const TabPreview: React.FC<TabPreviewProps> = ({
                 grow={false}
                 color="subdued"
                 paddingSize="none"
+                className="eui-textBreakWord"
                 css={getSplitPanelCss(euiTheme)}
               >
                 {tabPreviewData.status === TabStatus.RUNNING ? (
@@ -174,11 +175,11 @@ export const TabPreview: React.FC<TabPreviewProps> = ({
                       <EuiLoadingSpinner />
                     </EuiFlexItem>
                     <EuiFlexItem>
-                      <EuiText>{tabItem.label}</EuiText>
+                      <EuiText size="s">{tabItem.label}</EuiText>
                     </EuiFlexItem>
                   </EuiFlexGroup>
                 ) : (
-                  <EuiHealth color={tabPreviewData.status} textSize="m">
+                  <EuiHealth color={tabPreviewData.status} textSize="s">
                     {tabItem.label}
                   </EuiHealth>
                 )}

src/platform/packages/shared/kbn-unified-tabs/src/constants.ts

@@ -7,7 +7,7 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-export const MAX_TAB_LABEL_LENGTH = 500;
+export const MAX_TAB_LABEL_LENGTH = 120;
 
 export const MAX_TAB_WIDTH = 280;
 export const MIN_TAB_WIDTH = 112;