Improve the Ace Filter to only transform the code inside <ace> tags, leaving surrounding text untouched

Problem/Motivation

AceFilter::process() runs html_entity_decode() over the whole filtered text before it looks for any tag, then returns that transformed text. A text filter should only change the part it owns - the code inside an ... tag - and leave every other character exactly as the previous filters produced it. Decoding the entire string changes text that does not belong to this filter, which can turn intentionally entity-encoded characters produced by an earlier filter back into raw markup.

Steps to reproduce

1. Use a text format that runs the Ace Filter.
2. Enter body text containing entity-encoded characters outside any tag, e.g. <div>.
3. The rendered output shows <div> as raw markup instead of the literal text the author entered.

Proposed resolution

• Do not decode the whole text. Match the tags on the text as received.
• Decode only the extracted snippet content, which is handed to Ace as text through editor.getSession().setValue(), never as markup.
• Add kernel coverage: encoded markup outside a tag is returned unchanged; encoded content inside a tag is decoded for the editor.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ✅ Automated unit/functional testing coverage
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• The Ace Filter now transforms only the code inside tags and leaves the surrounding text unchanged.