Fix provisional node generation logic

[vish-cs]

We currently check the Node table to find missing nodes but this logic breaks since the import tool existence check also looks for typeOf properties in the Edge table. Once the provisional nodes are generated, the corresponding typeOf Edges are deleted by the following runs and never generated again causing import validation errors from the import tool. This PR checks the Edge table to generate provisional nodes.

Also, updated the manifest so generated provisional nodes are copied to GCS but not included in the current run causing a validation failure for a manual intervention to copy the generated nodes to be copied to the Schema repo.

[gemini-code-assist]

Code Review

This pull request introduces an optional output directory parameter for provisional node generation, handles missing Spanner dependencies gracefully, adds gcloud credential support, and updates download scripts and manifest configurations. The review feedback highlights a critical safety issue in download.sh where empty variables could lead to unintended deletions, suggests adding a timeout to the gcloud subprocess call to prevent hangs, and proposes optimizing the Spanner query to avoid an unnecessary table join.