Issue #3586369: "Global context" checkbox description renders a literal <br> instead of a line break

Description

Fixes issue #3586369: the "Global context" checkbox on the AI Context item edit form rendered a literal <br> instead of a line break in its description.

Root cause 1

AiContextScopeGlobal::getItemFormDescription() The method cast its TranslatableMarkup return value to (string), discarding the object that implements MarkupInterface — the only thing that tells Twig's autoescape not to Html::escape() the string. Fixed by returning the TranslatableMarkup directly.

Note on scope vs. the original issue text: since #3586369 was filed, issue #3586361 (closed) renamed this method from getFormDescription() to getItemFormDescription() and promoted it from a protected, single-caller method to a public method declared on AiContextScopeInterface, now implemented by 4 plugins (AiContextScopeBase default, AiContextScopeTag, AiContextScopeTargetEntity, AiContextScopeSiteSection). So the fix's real surface is larger than the original issue's proposed patch:

• AiContextScopeInterface::getItemFormDescription() — return type widened string → string|TranslatableMarkup; corrected a docblock that incorrectly claimed #description is filtered with Xss::filterAdmin() (it's actually plain Twig autoescape).
• AiContextScopeBase::getItemFormDescription() (default fallback) — same return-type widening, required because PHP checks return-type covariance against the immediate parent class, not just the interface (caught by PHPStan).
• AiContextScopeTag, AiContextScopeTargetEntity, AiContextScopeSiteSection — untouched; : string remains a valid covariant narrowing.

Root cause 2

AiContextItemForm.php (found during manual testing, not part of the original issue) buildForm() rebuilds the Global checkbox's #description to append a global-item-limit warning. It cast the (now-fixed) TranslatableMarkup to (string) and reinserted it via a :base placeholder in a new $this->t() call. Drupal's : placeholder always runs Html::escape(), even on MarkupInterface values — unlike @/%, which skip escaping when the value is MarkupInterface. That re-introduced the same literal-<br> bug through a second path. Fixed by keeping $base_description as the original TranslatableMarkup (no cast) and switching the placeholder from :base to @base in all three warning strings.

Test changes

• AiContextScopeGlobalTest::testItemFormDescriptionKeepsSaveWarning() — added a (string) cast around the getItemFormDescription() call, since the file has declare(strict_types=1) and assertStringContainsString() requires a string argument, not a TranslatableMarkup object.

Testing instructions

1. Install ai_context.
2. Go to /admin/config/ai/context/items/add (or edit any existing item).
3. Open the Context Scope section.
4. The description under the Global context checkbox now renders both line breaks correctly instead of literal <br>.

Checklist

• I have updated the MR title to use format: Issue #1234: My issue title.
• I have updated the MR description to include: Closes #1234
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

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Closes #3586369