agents-cli deploy has no way to pass CMEK (encryption_spec) for Agent Runtime, blocking deploys under constraints/gcp.restrictNonCmekServices

### What is your feature suggestion?

### Description
Deploying as ADK Agent to Agent Runtime(agents-cli deploy --deployment-target agent_runtime) fails in any GCP project that enforces the org policy constraints/gcp.restrictNonCmekServices, because the CLI never passes a KMS encryption key to the undersying create call.

### Reproduce

agents-cli scaffold create test-a2a-agent --agent adk --deployment-target agent_runtime --region us-central1 --cicd-runner skip
cd test-a2a-agent && agents-cli install
agents-cli deploy --project --region us-central1 --no-confirm-project

### Observed Error
Error: Agent Runtime create request failed — 400 FAILED_PRECONDITION: The request has violated one or more Org Policies: violations {
type: "constraints/gcp.restrictNonCmekServices"
subject: "orgpolicy:projects/"
description: "Constraint `constraints/gcp.restrictNonCmekServices` violated for `projects/` attempting to create a resource without specifying a KMS CryptoKey."
}

### Root cause

- agents-cli deploy --help exposes flags for --network-attachment, --service-account, --agent-gateway-*, etc., but has no --kms-key / --encryption-spec flag.
- The underlying SDK (vertexai.agent_engines._agent_engines.create(), in google_cloud_aiplatform==1.165.1 as vendored by the CLI) already accepts an encryption_spec: Optional[aip_types.EncryptionSpec] parameter — the capability exists in the SDK layer, it's just never threaded through google/agents/cli/deploy/cmd_deploy.py / agent_runtime.py.

### Requested fix

Add a --kms-key (or --encryption-spec) flag to agents-cli deploy for the agent_runtime target

### What will this enable you to do?

Any organization that mandates CMEK for Vertex AI resources (a common enterprise/regulated-industry policy) cannot deploy to Agent Runtime via agents-cli at all — there is no workaround short of bypassing the CLI and calling the

### Additional context

_No response_

Start with google/agents/cli/deploy/cmd_deploy.py and agent_runtime.py, then inspect vertexai.agent_engines._agent_engines.create() for its encryption_spec parameter. Run the reproduction command against a project enforcing constraints/gcp.restrictNonCmekServices and inspect agents-cli deploy --help. Done means an agent_runtime deploy can receive the KMS encryption key and satisfy that policy.

Written by the indexing model from the issue text.