Region needs to be configurable on every model

[babeal]

Is your feature request related to a problem? Please describe.

The assumption that the deployed location of the ADK agent will be the same for every model it invokes is incorrect. For example, if I have my service deployed in us central-1 and one of the sub agents needs to invoke Claude Sonnet 4, it doesn't work as that model isn't available in that region. Right now all models depend on the same environment variables making multi region calls very difficult.

Describe the solution you'd like

I need to be able to override model regions at the definition of the agent or model assignment.

Describe alternatives you've considered

N/A

Additional context

N/A