feat(vertex): add support for Vertex AI Gemini authentication and execution

Introduce Vertex AI Gemini integration with support for service account-based authentication, credential storage, and import functionality. Added new executor for Vertex AI requests, including execution and streaming paths, and integrated it into the core manager. Enhanced CLI with `--vertex-import` flag for importing service account keys.

internal/auth/vertex/vertex_credentials.go

@@ -0,0 +1,66 @@
+// Package vertex provides token storage for Google Vertex AI Gemini via service account credentials.
+// It serialises service account JSON into an auth file that is consumed by the runtime executor.
+package vertex
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/misc"
+	log "github.com/sirupsen/logrus"
+)
+
+// VertexCredentialStorage stores the service account JSON for Vertex AI access.
+// The content is persisted verbatim under the "service_account" key, together with
+// helper fields for project, location and email to improve logging and discovery.
+type VertexCredentialStorage struct {
+	// ServiceAccount holds the parsed service account JSON content.
+	ServiceAccount map[string]any `json:"service_account"`
+
+	// ProjectID is derived from the service account JSON (project_id).
+	ProjectID string `json:"project_id"`
+
+	// Email is the client_email from the service account JSON.
+	Email string `json:"email"`
+
+	// Location optionally sets a default region (e.g., us-central1) for Vertex endpoints.
+	Location string `json:"location,omitempty"`
+
+	// Type is the provider identifier stored alongside credentials. Always "vertex".
+	Type string `json:"type"`
+}
+
+// SaveTokenToFile writes the credential payload to the given file path in JSON format.
+// It ensures the parent directory exists and logs the operation for transparency.
+func (s *VertexCredentialStorage) SaveTokenToFile(authFilePath string) error {
+	misc.LogSavingCredentials(authFilePath)
+	if s == nil {
+		return fmt.Errorf("vertex credential: storage is nil")
+	}
+	if s.ServiceAccount == nil {
+		return fmt.Errorf("vertex credential: service account content is empty")
+	}
+	// Ensure we tag the file with the provider type.
+	s.Type = "vertex"
+
+	if err := os.MkdirAll(filepath.Dir(authFilePath), 0o700); err != nil {
+		return fmt.Errorf("vertex credential: create directory failed: %w", err)
+	}
+	f, err := os.Create(authFilePath)
+	if err != nil {
+		return fmt.Errorf("vertex credential: create file failed: %w", err)
+	}
+	defer func() {
+		if errClose := f.Close(); errClose != nil {
+			log.Errorf("vertex credential: failed to close file: %v", errClose)
+		}
+	}()
+	enc := json.NewEncoder(f)
+	enc.SetIndent("", "  ")
+	if err = enc.Encode(s); err != nil {
+		return fmt.Errorf("vertex credential: encode failed: %w", err)
+	}
+	return nil
+}