test: streamline backend tests and speed up validation

Remove obsolete and redundant assertions while preserving current behavior.
Optimize zlib-rs in dev/test builds and use eight concurrent CI test workers.
Use explicit time bases and synchronization for integration fixtures.

Validated 4322 tests, 72 Redis ACL tests, and ten stress iterations of 102
concurrency-related tests. Rustfmt, strict Clippy, and actionlint passed.

Co-authored-by: Codex <noreply@openai.com>

.github/workflows/_quality.yml

@@ -176,7 +176,7 @@ jobs:
           cargo nextest run \
             --cargo-metadata "$RUNNER_TEMP/nextest-cargo-metadata.json" \
             --binaries-metadata "$RUNNER_TEMP/nextest-binaries.json" \
-            --no-tests fail
+            --test-threads 8 --no-tests fail
 
       - name: Store tests with Redis ACL authentication
         working-directory: backend
@@ -195,7 +195,7 @@ jobs:
           cargo nextest run \
             --cargo-metadata "$RUNNER_TEMP/nextest-cargo-metadata.json" \
             --binaries-metadata "$RUNNER_TEMP/nextest-binaries.json" \
-            --no-tests fail -E 'package(=gateway-store) & test(/^redis::/)'
+            --test-threads 8 --no-tests fail -E 'package(=gateway-store) & test(/^redis::/)'
 
   frontend:
     if: inputs.run_frontend