feat(adkgo/deploy): add --env flag to agentengine deploy subcommand

[nuthalapativarun]

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Link to Issue or Description of Change

• Closes: Add custom list of Env Variables to Agent Engine deployment #848

Root Cause / Motivation

adkgo deploy agentengine passes a fixed set of environment variables to the Agent Engine deployment. Users that need custom env vars (e.g. VERTEX_AI_LOCATION, LOG_LEVEL, third-party API keys that are not secrets) have no way to inject them.

Change

Add a repeatable --env=KEY=VALUE flag to the agentengine deploy subcommand:

adkgo deploy agentengine \
  --env=VERTEX_AI_LOCATION=us-central1 \
  --env=LOG_LEVEL=debug \
  --region=... --project_name=...

On create: extra env vars are appended after the built-in defaults (GOOGLE_CLOUD_REGION, NUM_WORKERS, OTel flags) in DeploymentSpec.Env.

On update: if --env flags are provided, DeploymentSpec is set with defaults + extra vars and "spec.deployment_spec" is added to the update mask. If no --env flags are given, update behavior is unchanged.

Values containing = are handled correctly (URL=http://x?a=1&b=2 splits on the first = only). Entries without = return a clear error.

Testing Plan

Unit Tests:

• Added TestParseEnvVars in agentengine_test.go covering: nil input, single var, value-contains-equals, empty value, multiple vars, missing-equals error, empty-string error.
• All existing tests pass.

Manual End-to-End (E2E) Tests:
The flag wires correctly into the cobra command. The parseEnvVars function can be verified without a live GCP project; the proto field is identical to the existing hardcoded env var entries.

$ adkgo deploy agentengine --env=FOO=bar --env=BAZ=qux ...
# DeploymentSpec.Env will contain: GOOGLE_CLOUD_REGION, NUM_WORKERS, otel vars, FOO, BAZ

Checklist

• Read CONTRIBUTING.md
• Self-reviewed code
• Added tests proving the feature (parseEnvVars unit tests)
• New and existing tests pass
• Symmetric with the existing env var pattern in the proto request