fix(ci): only save test durations when tests pass

The save-durations job used `if: always()` which meant it would
run even when the test matrix failed, potentially caching duration
data from a failed/incomplete run. Changed to check
needs.test.result == 'success' so durations are only cached when
all test slices pass cleanly.

.github/workflows/tests.yml

@@ -129,7 +129,7 @@ jobs:
   # (including PRs) get balanced slicing.
   save-durations:
     needs: test
-    if: always() && github.ref == 'refs/heads/main'
+    if: needs.test.result == 'success' && github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     steps:
       - name: Download all slice durations
@@ -219,4 +219,4 @@ jobs:
         env:
           OPENROUTER_API_KEY: ""
           OPENAI_API_KEY: ""
-          NOUS_API_KEY: ""
+          NOUS_API_KEY: ""