IBX-12102: Fixed SiteAccess context being lost after fragment sub-requests

[tbialcz]

🎫 Issue	IBX-12102

Description:

In Page Builder, every URL generated after a page block (or any render_esi) lost the
SiteAccess prefix, e.g. // instead of /site_fr/.

Why it happened:

1. Each fragment sub-request overwrites the shared SiteAccess object, and nothing switched
   it back when the fragment finished. Fixed with a new kernel.finish_request listener
   that brings back the parent request's SiteAccess (same as Symfony does for its router).
2. On top of that, the SiteAccess sent to the fragment could be serialized before the
   URIElement matcher computed its URI elements, so after deserialization the matcher
   produced //. Now the elements are computed before serialization.

Origin: (1) is as old as the SiteAccess matching layer itself; (2) came with EZP-31810
(Sep 2020), which started serializing the raw, lazily-initialized property.

For QA:

1. Add a site_fr siteaccess (URI matching) and put
   {{ path('ibexa.url.alias', { locationId: 2 }) }} at the end of pagelayout.html.twig.
2. Edit the home page in Page Builder for site_fr: without a block, then add a block,
   publish and edit again.
3. Expected: the URL is always /site_fr/ (before the fix: // once a block is on the page).
4. Regression: frontend with fragments, back office, content preview across siteaccesses.

Documentation:

None.

[Steveb-p]

Makes sense.

Note (to self mostly) that #798 (6.0) will need slight adjustments once this lands.

[tbialcz]

Makes sense.

Note (to self mostly) that #798 (6.0) will need slight adjustments once this lands.

OK, once #798 lands, its stack pop does the same job, so my listener can go away on 6.0.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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