ci: harden workflow runners and update checkout action pins (#6445)

# Description of Changes

Add runner hardening and housekeeping across workflows.

- .github/dependabot.yml: add /docker/unoserver and /engine to
Dependabot update paths.
- .github/workflows/_runner-pick.yml and .github/workflows/build.yml:
add step-security/harden-runner (egress-policy: audit) to audit outbound
calls from runners.
- .github/workflows/ai-engine.yml, .github/workflows/aur-publish.yml,
.github/workflows/package-managers.yml: update actions/checkout usage to
the newer v6.0.2 reference.
- .github/workflows/package-managers.yml: minor YAML formatting tidy
(release types array).

These changes improve CI security by auditing egress, update the
checkout action to a newer release, and expand Dependabot coverage.

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(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](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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 run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

.github/workflows/build.yml

@@ -198,6 +198,11 @@ jobs:
       - dependency-review
     runs-on: ubuntu-latest
     steps:
+      - name: Harden the runner (Audit all outbound calls)
+        uses: step-security/harden-runner@ab7a9404c0f3da075243ca237b5fac12c98deaa5 # v2.19.3
+        with:
+          egress-policy: audit
+
       - name: Verify every required job passed (or was legitimately skipped)
         env:
           RESULTS: |