fix(litellm): provision Vertex sandbox runtime

[kywch]

Summary

Provision Vertex dependencies and ADC credentials inside sandboxed LiteLLM proxies.

Root cause

Vertex LiteLLM runs in a separate Daytona sandbox venv. #978 added google-cloud-aiplatform to BenchFlow host dependencies, but host packages are not visible in that proxy venv. The proxy also needed a local ADC credential-file path and explicit Vertex project/location.

Changes

• Install google-cloud-aiplatform only for sandbox Vertex routes.
• Stage ADC JSON as a sandbox-local credential file.
• Remove raw ADC JSON before proxy launch config serialization.
• Forward Vertex project/location to LiteLLM.

Scope

Complements #978. Does not change host provisioning or non-Vertex routes.

Validation

• 74 focused tests passed.
• git diff --check passed.