ci: add matrix strategy for spring-security and improve test report check logic

[Ludy87]

Description of Changes

• Introduced a new matrix axis spring-security in the GitHub Actions workflow to run tests with and without Spring Security features enabled.
• Removed duplicated build steps and unified them into a single Gradle task using the matrix value.
• Refactored the test report check step to dynamically iterate over expected directories using a Bash array, improving maintainability and readability.
• Ensured test report artifacts are uploaded regardless of test success, with clearer naming and a fallback if files are missing.

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have tested my changes locally. Refer to the Testing Guide for more details.

[Copilot]

Pull Request Overview

This PR enhances the CI workflow to capture additional test report outputs from the root project alongside the existing module directories.

• Adds checks for the root build/reports/tests/ and build/test-results/ directories before reporting.
• Extends the artifact upload step to include root-level test reports and problems outputs.
• Ensures multi-module Gradle builds preserve comprehensive test outputs in CI artifacts.

Comments suppressed due to low confidence (3)

.github/workflows/build.yml:99

• [nitpick] Consider adding a corresponding existence check for the root build/reports/problems/ directory in the missing_reports array to maintain consistency with the newly uploaded artifacts.

            build/reports/problems/

.github/workflows/build.yml:102

• [nitpick] You may want to include a check for the module's build/reports/problems/ directory in the missing_reports section to avoid missing artifacts silently.

            stirling-pdf/build/reports/problems/

.github/workflows/build.yml:57

• [nitpick] Consider using a glob pattern (e.g., /build/test-results/) to dynamically detect and upload all test report directories across modules instead of hardcoding each path.

          if [ ! -d "build/reports/tests/" ]; then