fix: mock client in php-unit test

[nabim777]

Description

Php unit tests were failing because Nextcloud 31+ changed( PR ) how clients work i.e Client constructor requires 6 parameters including ServerVersion, but our tests were still using the old way(without ServerVersion).
So, instead of creating complicated version-specific HTTP clients, here is simplified it to just mock the service that creates clients.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Updated CHANGELOG.md file

[github-actions]

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