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.

[Galius5136]

Took an independent look at fc842f14 and also ran the sandbox path against a real GCP project.

Local checks were clean: 74 touched tests, 261 wider provider tests, plus ruff, ruff format, and ty all passing.

Live E2E with Docker + google-vertex/gemini-2.5-flash also worked:

• sandbox-local LiteLLM proxy installed google-cloud-aiplatform
• ADC staged as a 0600 file
• raw GOOGLE_APPLICATION_CREDENTIALS_JSON removed from launch config
• correct Vertex project/location reached the proxy config
• real model calls completed successfully through the proxy

Two small non-blocking notes:

• This changes the effective Vertex location from LiteLLM's implicit default to BenchFlow's GOOGLE_CLOUD_LOCATION default (global). I verified global works for gemini-2.5-flash, but not live for anthropic-vertex/claude-sonnet-4-6, which is what the new tests use.
• If the sandbox LiteLLM install fails after ADC staging, the credential file remains in the runtime dir until sandbox teardown. I reproduced this with the fake sandbox; moving the install inside the existing cleanup scope would close that window.

From my side this looks good to merge.