feat(utils): default Vertex project and location from GCP metadata - #6703

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Drop the .env, it's cleaner (ADK should pick up project and location from GCP metadata server) #2685

Summary

When ADK runs on Google Cloud (Agent Engine / Cloud Run / GCE) without an explicit .env, fill unset:

• GOOGLE_CLOUD_PROJECT
• GOOGLE_CLOUD_LOCATION
• GOOGLE_GENAI_USE_ENTERPRISE (enterprise/Vertex mode)

from the instance metadata server.

Behavior:

• Explicit shell / .env values always win
• Does not force enterprise mode when an API key is present
• Off-GCP / local runs are unchanged (soft-fail metadata probe, cached)
• Applied after dotenv load, before GenAI client construction, and when checking enterprise mode

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

uv run pytest tests/unittests/utils/test_gcp_metadata.py tests/unittests/utils/test_env_utils.py -q
.......................                                                  [100%]
23 passed

Manual End-to-End (E2E) Tests:

• Local: leave project/location unset → no metadata defaults applied
• On GCP: omit .env Vertex identity vars → ADK picks project/region and enables enterprise mode

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

Additional context

cc @xuanyang15 @steren @klateefa @yeesian @wuliang229 @llalitkumarrr @GWeale — ready for review when you have a chance.