48472: Fix fatal error with celtic/lti 5.4.4 and later

[Saaweel]

Reported in Mantis 48472: updating the composer dependencies stops with a fatal error.

PHP Fatal error:  Declaration of ilLTITool::handleRequest(?bool $strictMode = null, bool $disableCookieCheck = false, bool $generateWarnings = false): void must be compatible with ceLTIc\LTI\Tool::handleRequest(?bool $strictMode = null, bool $disableCookieCheck = false, bool $generateWarnings = false): never

Since celtic/lti 5.4.4, Tool::handleRequest() is declared as never, and ilLTITool overrides it with void. The ^5.0.0 constraint allows any 5.x release, so composer update installs 5.4.7 and the setup stops while building the artifacts.

Fix

ilLTITool no longer overrides handleRequest(). The preparation of the request and the exit handling added in #12104 move unchanged to a new processRequest(), which ilAuthProviderLTI calls instead. As nothing overrides the library method anymore, its return type no longer matters, and the behaviour is the same as with the shipped 5.4.3.

There are no changes to composer.json or composer.lock.

A correction to the description of #12104: the exit handling relies on the behaviour of celtic/lti 5.4.0 and later, which stores the response before ending the request. 5.3.x prints it itself, so it must not be combined with that change. composer.lock ships 5.4.3.

Testing

• The changed classes were checked against celtic/lti 5.4.3 and 5.4.7: the class loads with both (5.4.7 gave the fatal error before), a valid launch returns control to ILIAS, an invalid one ends the request, and an error page without return URL is sent once.
• The same change was verified live on ILIAS 11.4 as LTI provider with 5.4.3 and 5.4.7 (48472: Fix blank page on LTI launches and fatal error with celtic/lti 5.4.4 #12184): launches redirect to the course, invalid signatures go back to the platform, and composer update celtic/lti builds all artifacts.
• php-cs-fixer reports no violations.