Add region support for ADK agent deployments

[HemasumanthDO]

Summary

• Add optional region field to agent config YAML (.gradient/agent.yml)
• Pass region through to CreateAgentWorkspaceInput and CreateAgentWorkspaceDeploymentInput API calls
• Add region field to API models, config manager, deploy service, and CLI

Usage

Users can set region in their config:

agent_name: my-agent
agent_environment: main
entrypoint_file: main.py
region: atl1

If omitted, the backend uses its default region.

Test plan

• Deploy with no region set in config (should use default, no behavior change)
• Deploy with region: atl1 in config (should pass region to API)
• Verify gradient agent configure still works without region
• Verify existing agent configs without region field still work

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