ci: bump codecov-action to v6 and fail CI on upload error (#690)

codecov-action below v6.0.2 verifies the codecov CLI against Codecov's
old Keybase key, which they bricked after migrating accounts, so uploads
now fail with "Could not verify signature". v6 (= 6.0.2) ships the
updated key.

Also sets fail_ci_if_error: true (the cookiecutter-scverse template
default) so a broken coverage upload surfaces as a CI failure instead of
passing silently.

Ref: codecov/codecov-action#1956

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: timtreis

.github/workflows/test-gpu.yml

@@ -108,21 +108,21 @@ jobs:
 
       - name: Upload test results
         if: ${{ !cancelled() && contains(matrix.env.name, 'stable') }}
-        uses: codecov/codecov-action@v5
+        uses: codecov/codecov-action@v6
         with:
           report_type: test_results
           files: test-data/test-results.xml
           use_oidc: false
-          fail_ci_if_error: false
+          fail_ci_if_error: true
           token: ${{ secrets.CODECOV_TOKEN }}
 
       - name: Upload coverage data
         if: contains(matrix.env.name, 'stable')
-        uses: codecov/codecov-action@v5
+        uses: codecov/codecov-action@v6
         with:
           files: test-data/coverage.xml
           use_oidc: false
-          fail_ci_if_error: false
+          fail_ci_if_error: true
           token: ${{ secrets.CODECOV_TOKEN }}
 
   test-prerelease: