feat: support VertexAI provider (#153)

* support: vertexai

fix

fix

set default for vertexai

added comment

fix

fix

* create schema

* fix README.md

* fix order

* added pupularity

* set tools if tools is exists

restore commentout

* fix comment

* set summarizer model

internal/llm/provider/vertexai.go

@@ -0,0 +1,34 @@
+package provider
+
+import (
+	"context"
+	"os"
+
+	"github.com/opencode-ai/opencode/internal/logging"
+	"google.golang.org/genai"
+)
+
+type VertexAIClient ProviderClient
+
+func newVertexAIClient(opts providerClientOptions) VertexAIClient {
+	geminiOpts := geminiOptions{}
+	for _, o := range opts.geminiOptions {
+		o(&geminiOpts)
+	}
+
+	client, err := genai.NewClient(context.Background(), &genai.ClientConfig{
+		Project:  os.Getenv("VERTEXAI_PROJECT"),
+		Location: os.Getenv("VERTEXAI_LOCATION"),
+		Backend:  genai.BackendVertexAI,
+	})
+	if err != nil {
+		logging.Error("Failed to create VertexAI client", "error", err)
+		return nil
+	}
+
+	return &geminiClient{
+		providerOptions: opts,
+		options:         geminiOpts,
+		client:          client,
+	}
+}