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: 23 calls in 6 files. The staticMethod.dynamicCall strict rule itself is enabled in the linked ibexa/phpstan PR; this PR keeps CI green once that is released.
Also fixes what the existing ibexa/phpstan rules report: 24 closures/arrow functions got return types (Ibexa.requireClosureReturnType), 3 converter constructors (Html5, Html5Edit, Html5Input) no longer read ConfigResolver parameters (Ibexa.noConfigResolverParametersInConstructor) — the resolver is now kept as a property and the custom stylesheets are resolved lazily on first use, with Xslt::addCustomStylesheets() extracted so the subclasses can call it after construction.
While fixing a pre-existing PHPStan argument.type finding in RichTextTransformerTest (honestly, not by baselining), one baseline entry became unmatched and was removed.
A follow-up commit extracts the shared logic of Html5, Html5Edit and Html5Input into a new ConfigResolverAwareXsltConverter base class: the initial Ibexa.noConfigResolverParametersInConstructor fix made those three classes near-identical, which SonarCloud correctly flagged as new duplicated code (67% on new lines, gate is 3%); this was genuinely new duplication introduced by this PR, not the known pre-existing-test-fixture false positive, so it was fixed rather than worked around.

Known CI failure (SonarCloud duplication gate): after that fix, SonarCloud still fails the "duplication on new code" gate (14.2%, threshold 3%), now entirely attributed to tests/lib/RichText/RendererTest.php (64% on its 28 new lines). This is the documented false positive: the flagged hasParameter/getParameter/warning/error/__get mock closures are pre-existing, near-identical blocks that already existed 2-3 times in this file on 6.0 before this branch; this PR only added a return-type declaration (and one self::/static conversion in commit 2) inside each occurrence, which makes SonarCloud attribute the whole pre-existing duplicated block as "new code". No new duplication was introduced. Per the established policy for this pattern, the test fixtures were intentionally left untouched rather than refactored to dodge the gate; all other checks (PHPStan, code style, unit, integration, browser tests) are green.

For QA:

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

Documentation:

No client-facing impact.

[sonarqubecloud]

Quality Gate failed

Failed conditions
14.2% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud

[konradoboza]

@bnowak do you feel sanities are needed here given refactor caused by ConfigResolver call in constructors?

[konradoboza]

After internal sync we decided to merge this one as-is to unblock further progress.