Refactor user & avatar (#33433)

1. better GetPossibleUserByID logic
2. fix some function name & comment typos
3. do not re-generate avatar if one exists

models/user/user_system_test.go

@@ -0,0 +1,32 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package user
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSystemUser(t *testing.T) {
+	u, err := GetPossibleUserByID(db.DefaultContext, -1)
+	require.NoError(t, err)
+	assert.Equal(t, "Ghost", u.Name)
+	assert.Equal(t, "ghost", u.LowerName)
+	assert.True(t, u.IsGhost())
+	assert.True(t, IsGhostUserName("gHost"))
+
+	u, err = GetPossibleUserByID(db.DefaultContext, -2)
+	require.NoError(t, err)
+	assert.Equal(t, "gitea-actions", u.Name)
+	assert.Equal(t, "gitea-actions", u.LowerName)
+	assert.True(t, u.IsGiteaActions())
+	assert.True(t, IsGiteaActionsUserName("Gitea-actionS"))
+
+	_, err = GetPossibleUserByID(db.DefaultContext, -3)
+	require.Error(t, err)
+}