Enable CI/CD integration testing with Secret Manager

[google-labs-jules]

• Modified pkg/secrets: Now prioritizes environment variables over Secret Manager. This allows local overrides and simplifies testing, while still falling back to Secret Manager in CI if env vars are missing but GCLOUD_PROJECT_ID is present.
• Updated deployment.yml: Authenticate with gcloud before the "Install, Build, Test" step. Passed GCLOUD_PROJECT_ID to the test environment.
• Refactored Unit Tests: Updated api_client_test.go, passage_test.go, and ask_test.go. "Success" scenarios now rely on the environment (or Secret Manager) to provide the API URL, effectively converting them to integration tests when running in CI. "Error" scenarios continue to use mocks.
• Added Database Test: Created pkg/app/database_integration_test.go to verify connectivity to USER_DATABASE_ID (Firestore) using a safe dummy user.

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PR created automatically by Jules for task 535107038277093355 started by @julwrites

[google-labs-jules]

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