test: use PSR container in InjectionMiddlewareTest

[vitormattos]

Summary

Fix InjectionMiddlewareTest on Nextcloud 36 after the removal of the deprecated OCP\IServerContainer interface.

The test instantiated OC\InitialStateService with a mock of OCP\IServerContainer. Nextcloud 36 removed that interface, while the current InitialStateService constructor accepts Psr\Container\ContainerInterface.

This PR replaces the obsolete mock with Psr\Container\ContainerInterface.

A repository-wide search found no other IServerContainer references in LibreSign.

Why

The PHPUnit jobs fail with:

PHPUnit\Framework\MockObject\Generator\UnknownTypeException:
Class or interface "OCP\IServerContainer" does not exist

This is triggered by the Nextcloud 36 server change, not by application behavior.

Scope

• Test-only compatibility fix.
• No production behavior changes.
• No public API changes.
• No backport planned; this is for main / Nextcloud 36 compatibility.

Validation

Expected to restore the failing InjectionMiddlewareTest cases in the PHPUnit matrix on Nextcloud 36.