test(antigravity): add missing unit tests for upstream and custom model_mapping

- Add GetAccessToken upstream branch tests (success/failure/empty/nil)
- Add mapAntigravityModel wildcard-target-equals-request edge case tests
- Add upstream account smart retry test case
- Add GeminiMessagesCompatService custom model_mapping and empty model tests

backend/internal/service/antigravity_token_provider_test.go

@@ -0,0 +1,97 @@
+//go:build unit
+
+package service
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAntigravityTokenProvider_GetAccessToken_Upstream(t *testing.T) {
+	provider := &AntigravityTokenProvider{}
+
+	t.Run("upstream account with valid api_key", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformAntigravity,
+			Type:     AccountTypeUpstream,
+			Credentials: map[string]any{
+				"api_key": "sk-test-key-12345",
+			},
+		}
+		token, err := provider.GetAccessToken(context.Background(), account)
+		require.NoError(t, err)
+		require.Equal(t, "sk-test-key-12345", token)
+	})
+
+	t.Run("upstream account missing api_key", func(t *testing.T) {
+		account := &Account{
+			Platform:    PlatformAntigravity,
+			Type:        AccountTypeUpstream,
+			Credentials: map[string]any{},
+		}
+		token, err := provider.GetAccessToken(context.Background(), account)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "upstream account missing api_key")
+		require.Empty(t, token)
+	})
+
+	t.Run("upstream account with empty api_key", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformAntigravity,
+			Type:     AccountTypeUpstream,
+			Credentials: map[string]any{
+				"api_key": "",
+			},
+		}
+		token, err := provider.GetAccessToken(context.Background(), account)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "upstream account missing api_key")
+		require.Empty(t, token)
+	})
+
+	t.Run("upstream account with nil credentials", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformAntigravity,
+			Type:     AccountTypeUpstream,
+		}
+		token, err := provider.GetAccessToken(context.Background(), account)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "upstream account missing api_key")
+		require.Empty(t, token)
+	})
+}
+
+func TestAntigravityTokenProvider_GetAccessToken_Guards(t *testing.T) {
+	provider := &AntigravityTokenProvider{}
+
+	t.Run("nil account", func(t *testing.T) {
+		token, err := provider.GetAccessToken(context.Background(), nil)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "account is nil")
+		require.Empty(t, token)
+	})
+
+	t.Run("non-antigravity platform", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformAnthropic,
+			Type:     AccountTypeOAuth,
+		}
+		token, err := provider.GetAccessToken(context.Background(), account)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "not an antigravity account")
+		require.Empty(t, token)
+	})
+
+	t.Run("unsupported account type", func(t *testing.T) {
+		account := &Account{
+			Platform: PlatformAntigravity,
+			Type:     AccountTypeAPIKey,
+		}
+		token, err := provider.GetAccessToken(context.Background(), account)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "not an antigravity oauth account")
+		require.Empty(t, token)
+	})
+}