test: run the firestore system tests in google cloud build

[danieljbruce]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request introduces a new Cloud Build configuration (cloudbuild.yaml) for running system tests and optional code coverage reporting in the handwritten/firestore directory. The feedback suggests a more robust and idiomatic way to run the locally installed nyc binary using npx --no-install instead of relying on a hardcoded internal path.

[gemini-code-assist]

Instead of hardcoding the internal path to the nyc executable (./node_modules/nyc/bin/nyc.js), which is fragile and dependent on the package's internal structure, use npx --no-install to run the locally installed binary. This is more idiomatic and robust.

    if npx --no-install nyc --version >/dev/null 2>&1; then
      npx nyc report || true
    else
      echo "nyc not found, skipping coverage report."
    fi