feat(config): add support for Fork in OAuth model mappings with alias handling

Implemented `Fork` flag in `ModelNameMapping` to allow aliases as additional models while preserving the original model ID. Updated the `applyOAuthModelMappings` logic, added tests for `Fork` behavior, and updated documentation and examples accordingly.

sdk/cliproxy/service_oauth_model_mappings_test.go

@@ -0,0 +1,58 @@
+package cliproxy
+
+import (
+	"testing"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/sdk/config"
+)
+
+func TestApplyOAuthModelMappings_Rename(t *testing.T) {
+	cfg := &config.Config{
+		OAuthModelMappings: map[string][]config.ModelNameMapping{
+			"codex": {
+				{Name: "gpt-5", Alias: "g5"},
+			},
+		},
+	}
+	models := []*ModelInfo{
+		{ID: "gpt-5", Name: "models/gpt-5"},
+	}
+
+	out := applyOAuthModelMappings(cfg, "codex", "oauth", models)
+	if len(out) != 1 {
+		t.Fatalf("expected 1 model, got %d", len(out))
+	}
+	if out[0].ID != "g5" {
+		t.Fatalf("expected model id %q, got %q", "g5", out[0].ID)
+	}
+	if out[0].Name != "models/g5" {
+		t.Fatalf("expected model name %q, got %q", "models/g5", out[0].Name)
+	}
+}
+
+func TestApplyOAuthModelMappings_ForkAddsAlias(t *testing.T) {
+	cfg := &config.Config{
+		OAuthModelMappings: map[string][]config.ModelNameMapping{
+			"codex": {
+				{Name: "gpt-5", Alias: "g5", Fork: true},
+			},
+		},
+	}
+	models := []*ModelInfo{
+		{ID: "gpt-5", Name: "models/gpt-5"},
+	}
+
+	out := applyOAuthModelMappings(cfg, "codex", "oauth", models)
+	if len(out) != 2 {
+		t.Fatalf("expected 2 models, got %d", len(out))
+	}
+	if out[0].ID != "gpt-5" {
+		t.Fatalf("expected first model id %q, got %q", "gpt-5", out[0].ID)
+	}
+	if out[1].ID != "g5" {
+		t.Fatalf("expected second model id %q, got %q", "g5", out[1].ID)
+	}
+	if out[1].Name != "models/g5" {
+		t.Fatalf("expected forked model name %q, got %q", "models/g5", out[1].Name)
+	}
+}