Enable failing linters in GitLabCI and fix the reported errors

Problem/Motivation

Several linters in the GitLab CI pipeline (cspell, eslint, phpcs, phpstan, phpstan previous major, and stylelint) are running but not configured to fail the pipeline when they detect problems. This allows lint errors to accumulate unnoticed, reducing code quality and making it harder to trace when issues were introduced.

Proposed resolution

• Update the GitLab CI configuration so each linter is allowed to fail only when appropriate, and otherwise fails the pipeline.
• Fix the existing violations reported by these linters.
• Ensure future merge requests surface lint failures clearly.

Remaining tasks

• Review the current CI configuration for each linter.
• Enable required failure behavior.
• Run the linters locally or via CI to gather the current set of violations.
• Create or update patches to fix the reported issues.
• Request code review.