[BUGFIX:13.1] Prevent c:0 variant and content leakage on fe_group-restricted pages

[dkd-kaehm]

Fixes two interrelated bugs in the page indexing flow that cause
access-protected pages to be indexed with incorrect Solr documents:

• Bug 1 (PageIndexer::index): the findUserGroups indexer request
  collects fe_group values from all content rendered for that
  request, including global-template content (footer/nav) which
  typically has fe_group=0. Group 0 then reaches accessGroups and
  triggers an unwanted c:0 variant via indexPage, making the page
  findable without the required access group. Group 0 is now
  filtered out when the page has fe_group > 0, and the page's own
  group is ensured to be present so restricted-but-eligible users
  can still find the page.
• Bug 2 (FrontendGroupsModifier): the event listener faked
  pageUserGroup into the frontend groups unconditionally. During
  the c:0 indexer request this granted access to fe_group-restricted
  content elements, leaking protected bodytext into the c:0 Solr
  document. pageUserGroup is now only faked in when userGroup > 0;
  the c:0 request runs with anonymous groups.

The new fixture mirrors the production scenario: a protected page
(fe_group=1) with a protected CE (fe_group=1) alongside a footer/nav
CE (pid=1, fe_group=0) rendered via a second CONTENT object outside
the TYPO3SEARCH markers — the same shape global template content has
in production. Three existing data-provider yields had been silently
encoding the bug as expected ('2:1/c:0' on a page with fe_group=1)
and are corrected to '2:1/c:1', the semantically correct access
variant.

Files:

• Classes/IndexQueue/PageIndexer.php
  • In index(): read pageUserGroup from the page record (via
    enablecolumns.fe_group). For protected pages, filter group 0
    from accessGroups and ensure pageUserGroup is in the list.
• Classes/EventListener/PageIndexer/FrontendGroupsModifier.php
  • Add a userGroup > 0 guard to the pageUserGroup faking block.
• Tests/Integration/IndexQueue/PageIndexerTest.php
  • Added data-provider yield "protected page: c:0 must not contain
    same-group protected content (isolation bug)" expecting only c:1.
  • Corrected three protected-page yields ("protected page",
    "protected page with protected content", "translation of
    protected page with protected content"):
    expectedAccessFieldValues '2:1/c:0' -> '2:1/c:1'.
• Tests/Integration/IndexQueue/Fixtures/can_index_protected_page_with_public_and_same_group_protected_content.csv
  • New fixture; sys_template inlined to render bodytext and
    include the second CONTENT object outside TYPO3SEARCH markers.
• Documentation/Releases/solr-release-13-1.rst
  • Added a Release 13.1.2 section with this fix as the first entry.

Partially ports: #4559
Closes: #4642