cloud run docs: Add authentication prerequisites to deployment guide

[hari-silvers]

ISSUE NO - 1271

Problem

it's not that much of change but the deployment guide (deploy.sh) uses gcloud commands but does not mention
authentication steps. Users who are not authenticated will face failures
when running the script.

Proposed Fix

Add a Prerequisites section to the README after the Introduction that covers:

• Running gcloud auth login
• Running gcloud auth application-default login
• Enabling required APIs

The proposed changes rendered in the A2UI\samples\agent\adk\gemini_enterprise\cloud_run\README.md is shown below:

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on fork, I have verified that scripts/e2e_test.sh passes.

If you need help, consider asking for advice on the discussion board.

[gemini-code-assist]

Code Review

This pull request introduces a Prerequisites section to the README for deploying a Gemini-based agent on Cloud Run, including authentication steps and API enablement. The review feedback highlights that the requirement for Application Default Credentials is misleading for this deployment context and suggests including additional necessary APIs, such as Artifact Registry and IAM, to ensure the workflow functions correctly.