Add MongoDB storage library and update acceptance tests with deterministic stubs

- Created StellaOps.Notify.Storage.Mongo project with initial configuration.
- Added expected output files for acceptance tests (at1.txt to at10.txt).
- Added fixture input files for acceptance tests (at1 to at10).
- Created input and signature files for test cases fc1 to fc5.

.gitea/workflows/export-ci.yml

@@ -28,6 +28,9 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+
+      - name: Task Pack offline bundle fixtures
+        run: python3 scripts/packs/run-fixtures-check.sh
         with:
           fetch-depth: 0