refactor: add ValidateSession gRPC and streamline test setup

- Add ValidateSession gRPC method for proxy-side user validation
- Move group access validation from REST callback to gRPC layer
- Capture user info in access logs via CapturedData mutable pointer
- Create validate_session_test.go for gRPC validation tests
- Simplify auth_callback_integration_test.go to create accounts
  programmatically instead of using SQL file
- SQL test data file now only used by validate_session_test.go

management/internals/shared/grpc/proxy_group_access_test.go

@@ -68,6 +68,10 @@ func (m *mockReverseProxyManager) GetProxyByID(ctx context.Context, accountID, r
 	return nil, nil
 }
 
+func (m *mockReverseProxyManager) GetProxyIDByTargetID(_ context.Context, _, _ string) (string, error) {
+	return "", nil
+}
+
 type mockUsersManager struct {
 	users map[string]*types.User
 	err   error