ci: gate backend builds and cache priming (#7405)

# Description of Changes

- Added a dedicated backend path filter for Java and Gradle-related
changes.
- Gated backend builds on backend-relevant file changes.
- Limited Gradle cache priming to project changes.
- Made Docker image tests resilient to skipped upstream jobs.
- Included the Gradle cache prime job in the final CI status check.
- Removed the unnecessary build dependency from the license check.


---

## 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

@@ -37,6 +37,7 @@ jobs:
     timeout-minutes: 3
     outputs:
       build: ${{ steps.changes.outputs.build }}
+      backend: ${{ steps.changes.outputs.backend }}
       project: ${{ steps.changes.outputs.project }}
       openapi: ${{ steps.changes.outputs.openapi }}
       frontend: ${{ steps.changes.outputs.frontend }}
@@ -65,6 +66,7 @@ jobs:
       name: ci-unsigned
       deployment: false
     name: Prime shared Gradle cache
+    if: needs.files-changed.outputs.project == 'true'
     needs: [files-changed]
     runs-on: ubuntu-latest
     timeout-minutes: 15
@@ -118,6 +120,7 @@ jobs:
           key: ${{ steps.gradle-cache-key.outputs.key }}
 
   build:
+    if: needs.files-changed.outputs.backend == 'true'
     needs: [files-changed, gradle-cache-prime]
     permissions:
       actions: read
@@ -196,7 +199,7 @@ jobs:
 
   check-licence:
     if: needs.files-changed.outputs.build == 'true'
-    needs: [files-changed, build, gradle-cache-prime]
+    needs: [files-changed, gradle-cache-prime]
     permissions:
       contents: read
     uses: ./.github/workflows/check-licence.yml
@@ -215,7 +218,14 @@ jobs:
       docker-base-changed: ${{ needs.files-changed.outputs.docker-base }}
 
   test-build-docker-images:
-    if: github.event_name == 'pull_request' && needs.files-changed.outputs.project == 'true'
+    if: |
+      always() &&
+      github.event_name == 'pull_request' &&
+      needs.files-changed.outputs.project == 'true' &&
+      contains(fromJSON('["success", "skipped"]'), needs.gradle-cache-prime.result) &&
+      contains(fromJSON('["success", "skipped"]'), needs.build.result) &&
+      contains(fromJSON('["success", "skipped"]'), needs.check-generateOpenApiDocs.result) &&
+      contains(fromJSON('["success", "skipped"]'), needs.check-licence.result)
     needs:
       [
         files-changed,
@@ -321,6 +331,7 @@ jobs:
     if: always()
     needs:
       - files-changed
+      - gradle-cache-prime
       - build
       - db-migration-test
       - check-generateOpenApiDocs
@@ -348,6 +359,7 @@ jobs:
         env:
           RESULTS: |
             files-changed=${{ needs.files-changed.result }}
+            gradle-cache-prime=${{ needs.gradle-cache-prime.result }}
             build=${{ needs.build.result }}
             db-migration-test=${{ needs.db-migration-test.result }}
             check-generateOpenApiDocs=${{ needs.check-generateOpenApiDocs.result }}