Authenticate Secret Manager with GCLOUD_SA_KEY

[google-labs-jules]

Updated pkg/secrets to support explicit authentication via GCLOUD_SA_KEY environment variable, enabling tests to run in authenticated environments. Also fixed test isolation in pkg/app to prevent accidental Secret Manager access during unit tests.

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PR created automatically by Jules for task 15375844695840067266 started by @julwrites

[google-labs-jules]

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