fix(EuiMarkdownEditor): update types to allow UI plugins to not have a button

[tkajtoch]

Summary

Resolves #9557

This PR updates the EuiMarkdownEditorUiPlugin type to make the button property optional per our documentation, and applies necessary adjustments to the internal EuiMarkdownEditorToolbar.

API Changes

component / parent	prop / child	change	description
EuiMarkdownEditor	uiPlugins[].button	button is now optional	The button property is now correctly marked as optional in the type definition. When unset, the button won't be rendered in the toolbar, allowing for formatting-only plugins.

Screenshots

N/A

Impact Assessment

Note: Most PRs should be tested in Kibana to help gauge their Impact before merging.

• 🔴 Breaking changes — What will break? How many usages in Kibana/Cloud UI are impacted?
• 💅 Visual changes — May impact style overrides; could require visual testing. Explain and estimate impact.
• 🧪 Test impact — May break functional or snapshot tests (e.g., HTML structure, class names, default values).
• 🔧 Hard to integrate — If changes require substantial updates to Kibana, please stage the changes and link them here.

Impact level: 🟢 None, because the type becomes less strict.

Release Readiness

• Documentation: {link to docs page(s)}
• Figma: {link to Figma or issue}
• Migration guide: {steps or link, for breaking/visual changes or deprecations}
• Adoption plan (new features): {link to issue/doc or outline who will integrate this and where}

QA instructions for reviewer

• Type correctness
  • Checkout this PR, install dependencies and confirm that the EuiMarkdownEditorUiPlugin type allows for skipping the button property.
• UI correctness
  • Go to https://eui.elastic.co/pr_9634/docs/components/editors-and-syntax/markdown/editor/#base-editor
  • Update the interactive example to the following:

    import React, { useCallback, useState } from 'react';
    import { EuiMarkdownEditor, getDefaultEuiMarkdownPlugins } from '@elastic/eui';
    
    const { uiPlugins } = getDefaultEuiMarkdownPlugins();
    uiPlugins.push({
     name: 'PR test',
     formatting: { prefix: '!{blah[', suffix: ']()}' },
     helpText: 'Thanks for testing my PR!'
    })
    
    export default () => {
     const [value, setValue] = useState('## Hello, World!');
     const [messages, setMessages] = useState([]);
     const onParse = useCallback((err, { messages, ast }) => {
       setMessages(err ? [err] : messages);
     }, []);
    
     const [isReadOnly, setIsReadOnly] = useState(false);
    
     const onChange = (e) => {
       setIsReadOnly(e.target.checked);
     };
    
     return (
       <EuiMarkdownEditor
         aria-label="EUI markdown editor demo"
         placeholder="Your markdown here..."
         value={value}
         onChange={setValue}
         height={400}
         onParse={onParse}
         errors={messages}
         readOnly={isReadOnly}
         initialViewMode="viewing"
         uiPlugins={uiPlugins}
       />
     );
    };

  • Confirm that the toolbar doesn't render any extra icons when compared with the default example and that TypeError: Cannot read properties of undefined (reading 'label') is not thrown (it is on main)

Checklist before marking Ready for Review

• Filled out all sections above
• QA: Tested light/dark modes, high contrast, mobile, Chrome/Safari/Edge/Firefox, keyboard-only, screen reader
• QA: Tested in CodeSandbox and Kibana
• QA: Tested docs changes
• Tests: Added/updated Jest, Cypress, and VRT
• Changelog: Added changelog entry
• Breaking changes: Added breaking change label (if applicable)

Reviewer checklist

• Approved Impact Assessment — Acceptable to merge given the consumer impact.
• Approved Release Readiness — Docs, Figma, and migration info are sufficient to ship.

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 573d629
• Documentation website
• Storybook

cc @tkajtoch

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 573d629

cc @tkajtoch

[mgadewoll]

🟢 Changes are LGTM. Tested the update locally - the type for button works correctly and the button can be rendered optionally as expected.