ci: bump codecov-action to v6 and fail CI on upload error (#1142)

codecov-action@v5 verifies the codecov CLI against Codecov's old Keybase
key, which they bricked after migrating accounts, so the upload step now
fails with "Could not verify signature". v6 (= 6.0.2) ships the updated
key.

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

Ref: codecov/codecov-action#1956

Author: timtreis

.github/workflows/test.yaml

@@ -61,9 +61,10 @@ jobs:
               run: |
                   uv run pytest --cov --color=yes --cov-report=xml -n auto --dist worksteal
             - name: Upload coverage to Codecov
-              uses: codecov/codecov-action@v5
+              uses: codecov/codecov-action@v6
               with:
                   name: coverage
                   verbose: true
+                  fail_ci_if_error: true
               env:
                   CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}