Vertex (Anthropic-on-Vertex) doesn't share the chat/messages wire shapes:
the model travels in a rawPredict path and the proxy mints the service
account's OAuth token. Add a Vertex client method that posts
/v1/projects/<project>/locations/<region>/publishers/anthropic/models/<model>:rawPredict
with the Vertex anthropic_version body, and wire it into the matrix as a
path-routed provider (created without a models array). It is keyed off
GOOGLE_VERTEX_SA_BASE64 + GOOGLE_VERTEX_PROJECT (region defaults to
"global", model to a pinned claude snapshot, both overridable).
Also bump the e2e workflow runner to ubuntu-latest and add the Vertex
secrets.
The e2e suite (build tag `e2e`) stands up the combined server + proxy +
client in Docker and drives live chat-completions, so it is slow and needs
provider credentials. Gate it out of normal CI (it already is, via the
build tag) and run it on demand via workflow_dispatch. Provider scenarios
skip when their secret is unset, so it degrades gracefully.
