[e2e] Add Vertex to the provider matrix; run e2e on ubuntu-latest

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.

.github/workflows/agent-network-e2e.yml

@@ -17,7 +17,7 @@ concurrency:
 jobs:
   e2e:
     name: Agent Network E2E
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-latest
     timeout-minutes: 45
     steps:
       - name: Checkout code
@@ -44,4 +44,10 @@ jobs:
           CLOUDFLARE_TOKEN: ${{ secrets.E2E_CLOUDFLARE_TOKEN }}
           AWS_BEARER_TOKEN_BEDROCK: ${{ secrets.E2E_AWS_BEARER_TOKEN_BEDROCK }}
           AWS_REGION: ${{ secrets.E2E_AWS_REGION }}
+          # Vertex (Anthropic-on-Vertex): SA + project required; region defaults
+          # to "global", model to a pinned claude snapshot.
+          GOOGLE_VERTEX_SA_BASE64: ${{ secrets.E2E_GOOGLE_VERTEX_SA_BASE64 }}
+          GOOGLE_VERTEX_PROJECT: ${{ secrets.E2E_GOOGLE_VERTEX_PROJECT }}
+          GOOGLE_VERTEX_REGION: ${{ secrets.E2E_GOOGLE_VERTEX_REGION }}
+          GOOGLE_VERTEX_MODEL: ${{ secrets.E2E_GOOGLE_VERTEX_MODEL }}
         run: go test -tags e2e -timeout 40m -v ./e2e/...