Feat: Add Infection mutation testing

[UmairRathore]

Resolves: #8000

📝 Summary

Adds Infection mutation testing to LibreSign and integrates it into GitHub Actions.

This PR:

• adds Infection 0.35.x as an isolated Composer dependency under vendor-bin/infection
• adds infection.json5 covering the lib/ source tree while excluding migrations
• adds the required Nextcloud bootstrap so Infection can resolve both Nextcloud and LibreSign classes
• adds a composer mutation:test command
• adds a GitHub Actions workflow that installs Nextcloud, enables LibreSign, and runs mutation testing on pull requests
• enforces minimum MSI and Covered Code MSI thresholds of 80%
• uploads the Infection log when the mutation-testing job fails

While validating the full mutation suite, ProcessSignaler::stopPid() exposed a real OS side effect during mutation testing: mutations of the invalid-PID guard could allow PID 0 to reach posix_kill(), which signals the caller's process group.

To make this code safely mutation-testable, the OS signaling operation is isolated behind a protected sendSignal() boundary. The focused unit tests verify that invalid PIDs never reach this boundary and that valid PIDs are delegated correctly, without sending real process signals.

No Infection exclusions or mutator-specific ignores were added for this case.

🧪 How to test

Run the focused ProcessSignaler unit tests:

composer test:unit -- --filter ProcessSignalerTest

Run mutation testing:

composer mutation:test -- --no-progress --no-interaction

The focused ProcessSignaler validation produced:

• 5 tests
• 8 assertions
• 11/11 mutants killed
• 100% Covered Code MSI

A fresh end-to-end Infection run against the current checkout completed with:

• 15,180 mutations generated
• 7,305 killed
• 1,779 escaped
• 100% Mutation Code Coverage
• 81% Covered Code MSI

The configured CI thresholds are:

MSI >= 80%
Covered Code MSI >= 80%

⚙️ API / Back-end changes

• Added Infection mutation-testing configuration
• Added isolated Infection Composer dependencies
• Added Nextcloud/LibreSign bootstrap for mutation testing
• Added GitHub Actions mutation-testing workflow
• Added minimum MSI and Covered Code MSI thresholds
• Isolated the process-signaling OS boundary for safe mutation testing
• Added focused unit coverage for the signaling boundary
• Capabilities updated — not applicable
• API documentation updated — not applicable

✅ Checklist

• I have read and followed the contribution guide.
• Focused ProcessSignaler tests pass
• Focused ProcessSignaler mutation testing passes with 100% MSI
• Fresh end-to-end mutation run completed
• git diff --check passes
• Changes are limited to mutation-testing infrastructure and the required process-signaling test seam

🤖 AI

• The content of this PR was partially or fully generated using AI

[vitormattos]

Have a pattern to get the minimum supported PHP version from appinfo/info.xml.

Could you reuse the same approach here instead of hardcoding PHP 8.3?

- name: Get php version
  id: php-versions
  uses: icewind1991/nextcloud-version-matrix@8a7bac6300b2f0f3100088b297995a229558ddba # v1.3.2.3.1.3.2
  with:
    filename: ${{ env.APP_NAME }}/appinfo/info.xml

Then:

php-version: ${{ steps.php-versions.outputs.php-min }}

This way, when the minimum supported PHP version changes in appinfo/info.xml, the Infection workflow will follow it automatically.

[UmairRathore]

Updated to derive the minimum supported PHP version from appinfo/info.xml using the existing version-matrix pattern. Thanks!

[vitormattos]

We already use a more complete Nextcloud installation setup in the existing PHPUnit workflows.

Could you follow the same pattern here instead of relying on the default values?

      - name: Set up Nextcloud
        env:
          DB_PORT: 4444
        run: |
          mkdir data
          ./occ maintenance:install \
            --verbose \
            --database=sqlite \
            --database-name=nextcloud \
            --database-host=127.0.0.1 \
            --database-port=$DB_PORT \
            --database-user=root \
            --database-pass=rootpassword \
            --admin-user admin \
            --admin-pass admin

This keeps the Infection workflow equals to the same setup way used at workflows with PHPUnit and makes the CI environment explicit instead of depending on maintenance:install defaults.

[UmairRathore]

Updated the Nextcloud setup to match the explicit SQLite installation pattern used by the PHPUnit workflows. Thanks!

[vitormattos]

Thanks a lot by your contribution!
Before approve to execute the CI, only have small changes to do.

[UmairRathore]

Thanks for the review! I have addressed both requested changes. The Infection workflow now follows the existing PHPUnit patterns for PHP-version detection and Nextcloud installation. Appreciate the guidance.