[Tests] Fixed SiteAccessLimitationMapperTest generate mock

[alongosz]

Question	Answer
JIRA issue	n/a
Type	bug
Target Ibexa version	v3.3+
BC breaks	no

SiteAccessKeyGeneratorInterface::generate(string) returns string, not SiteAccessKeyGenerator as previously mocked.
This results in a silent error:

Method generate may not return value of type EzSystems\EzPlatformAdminUi\Siteaccess\SiteAccessKeyGenerator, its return declaration is ": string"

Noticed while working on the other PR. Worth fixing to avoid spending time investigating if the failure is related or not to the current changes.

Checklist:

• Provided PR description.
• Tested the solution manually.
• Provided automated test coverage.
• Checked that target branch is set correctly (main for features, the oldest supported for bugs).
• Ran PHP CS Fixer for new PHP code (use $ composer fix-cs).
• Asked for a review

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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