[Discover][A11y] Add underline to search result highlights (elastic#228086)

Closes elastic#214592

This PR adds `text-decoration: dotted underline` to search result
highlights due to accessibility reasons.

<img width="1427" height="493" alt="Screenshot 2025-07-15 at 21 08 59"
src="https://github.com/user-attachments/assets/c9edb19a-7a59-4aef-b8fe-91f424f62abf"
/>

<img width="542" height="774" alt="Screenshot 2025-07-15 at 21 07 50"
src="https://github.com/user-attachments/assets/e7be0fd1-78d0-46a7-abc4-ec6f4652c7ab"
/>

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] 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)
- [ ] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

Does this PR introduce any risks? For example, consider risks like hard
to test bugs, performance regression, potential of data loss.

Describe the risk, its severity, and mitigation for each identified
risk. Invite stakeholders and evaluate how to proceed before merging.

- [ ] [See some risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx)
- [ ] ...

(cherry picked from commit 24e9dc2)

Author: akowalska622

src/core/public/styles/rendering/_base.scss

@@ -78,3 +78,18 @@
     --kbnAppHeadersOffset: var(--euiFixedHeadersOffset, 0);
   }
 }
+
+// Styles applied to the span.ffArray__highlight from FieldFormat class that is used to visually distinguish array delimiters when rendering array values as HTML in Kibana field formatters
+.ffArray__highlight {
+  color: $euiColorMediumShade;
+}
+
+// Styles applied to the span.ffSearch__highlight from FieldFormat class that is used to visually distinguish highlighted string values when rendering string values as HTML in Kibana field formatters
+.ffSearch__highlight {
+  text-decoration: dotted underline;
+}
+
+// Styles applied to the span.ffString__emptyValue from FieldFormat class that is used to visually distinguish empty string values when rendering string values as HTML in Kibana field formatters
+.ffString__emptyValue {
+  color: $euiColorDarkShade;
+}

src/platform/packages/shared/kbn-discover-contextual-components/src/data_types/logs/components/utils/truncate_preserve_highlight_tags.ts

@@ -9,7 +9,7 @@
 
 function extractTextAndMarkTags(html: string) {
   const markTags: string[] = [];
-  const cleanText = html.replace(/<\/?mark>/g, (match) => {
+  const cleanText = html.replace(/<\/?mark[^>]*>/g, (match) => {
     markTags.push(match);
     return '';
   });

src/platform/plugins/shared/field_formats/common/converters/string.test.ts

@@ -124,6 +124,6 @@ describe('String Format', () => {
           },
         })
       )
-    ).toBe('<mark>&lt;img /&gt;</mark>');
+    ).toBe('<mark class="ffSearch__highlight">&lt;img /&gt;</mark>');
   });
 });

src/platform/plugins/shared/field_formats/common/utils/highlight/html_tags.ts

@@ -9,6 +9,6 @@
 
 // These are the html tags that will replace the highlight tags.
 export const htmlTags = {
-  pre: '<mark>',
+  pre: '<mark class="ffSearch__highlight">',
   post: '</mark>',
 };