ci(deps): bump codecov/codecov-action from 5 to 6

v6 moves the action runtime to Node 24, which GitHub-hosted
`ubuntu-latest` runners support natively. The `action.yml` diff
between v5.5.4 and v6.0.0 is purely an internal
`actions/github-script` SHA bump (v7 → v8) — the `token` input we
pass is byte-identical.

Tag: https://github.com/codecov/codecov-action/tree/v6
Changelog: https://github.com/codecov/codecov-action/blob/main/CHANGELOG.md

Author: tony

.github/workflows/tests.yml

@@ -82,7 +82,7 @@ jobs:
           COV_CORE_SOURCE: .
           COV_CORE_CONFIG: .coveragerc
           COV_CORE_DATAFILE: .coverage.eager
-      - uses: codecov/codecov-action@v5
+      - uses: codecov/codecov-action@v6
         with:
           token: ${{ secrets.CODECOV_TOKEN }}