IBX-12654: Added ibexa/phpstan and fixed dynamic calls to static methods

[bnowak]

🎫 Issue	IBX-12654

Related PRs:

• IBX-12654: Enabled the PHPStan strict rule for dynamic calls to static methods phpstan#16

Description:

Adds ibexa/phpstan as a dev dependency and includes its extension in the PHPStan configuration.
Replaces dynamic calls to static methods ($this->assertX(), static test helpers, ...) with self::/ClassName:: calls: 2 calls in 2 files. The staticMethod.dynamicCall strict rule itself is enabled in the linked ibexa/phpstan PR; this PR keeps CI green once that is released.
No issues reported by ibexa/phpstan's existing rules, so no third commit was needed.

Also fixes a CI-only issue: ibexa/phpstan requires ibexa/doctrine-schema back (this package), so a detached PR-merge checkout could not derive a branch-based Composer root version, breaking resolution. Fixed by adding ibexa/gh-workflows/actions/setup-composer-root-version@main before every composer-install@main step in backend-ci.yaml (same fix already applied to ibexa/core, which has the identical self-reference).

Judgment call: ibexa/doctrine-schema does not require ibexa/core directly, so requiring ibexa/phpstan (which requires ibexa/core) failed Composer's minimum-stability check for the transitively-required ibexa/core dev version. Rather than loosening this package's stability settings, added "ibexa/core": "~6.0.x-dev" directly to require-dev, so Composer has an explicit dev-stability constraint to resolve against without needing minimum-stability/prefer-stable.

For QA:

No functional change. CI (PHPStan, code style, unit tests) is the verification.

Documentation:

No client-facing impact.

[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

See analysis details on SonarQube Cloud