feat: map claude-haiku-4-5 variants to claude-sonnet-4-6

Update model mapping target for claude-haiku-4-5 and
claude-haiku-4-5-20251001 from claude-sonnet-4-5 to claude-sonnet-4-6.
Includes migration script, default constants, and test updates.

backend/internal/domain/constants.go

@@ -82,8 +82,8 @@ var DefaultAntigravityModelMapping = map[string]string{
 	"claude-opus-4-5-20251101":   "claude-opus-4-6-thinking", // 迁移旧模型
 	"claude-sonnet-4-5-20250929": "claude-sonnet-4-5",
 	// Claude Haiku → Sonnet（无 Haiku 支持）
-	"claude-haiku-4-5":          "claude-sonnet-4-5",
-	"claude-haiku-4-5-20251001": "claude-sonnet-4-5",
+	"claude-haiku-4-5":          "claude-sonnet-4-6",
+	"claude-haiku-4-5-20251001": "claude-sonnet-4-6",
 	// Gemini 2.5 白名单
 	"gemini-2.5-flash":               "gemini-2.5-flash",
 	"gemini-2.5-flash-image":         "gemini-2.5-flash-image",

backend/internal/service/antigravity_model_mapping_test.go

@@ -57,16 +57,16 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 			expected:       "claude-opus-4-6-thinking",
 		},
 		{
-			name:           "默认映射 - claude-haiku-4-5 → claude-sonnet-4-5",
+			name:           "默认映射 - claude-haiku-4-5 → claude-sonnet-4-6",
 			requestedModel: "claude-haiku-4-5",
 			accountMapping: nil,
-			expected:       "claude-sonnet-4-5",
+			expected:       "claude-sonnet-4-6",
 		},
 		{
-			name:           "默认映射 - claude-haiku-4-5-20251001 → claude-sonnet-4-5",
+			name:           "默认映射 - claude-haiku-4-5-20251001 → claude-sonnet-4-6",
 			requestedModel: "claude-haiku-4-5-20251001",
 			accountMapping: nil,
-			expected:       "claude-sonnet-4-5",
+			expected:       "claude-sonnet-4-6",
 		},
 		{
 			name:           "默认映射 - claude-sonnet-4-5-20250929 → claude-sonnet-4-5",

backend/migrations/075_map_haiku45_to_sonnet46.sql

+-- Map claude-haiku-4-5 variants target from claude-sonnet-4-5 to claude-sonnet-4-6
+--
+-- Only updates when the current target is exactly claude-sonnet-4-5.
+
+-- 1. claude-haiku-4-5
+UPDATE accounts
+SET credentials = jsonb_set(credentials, '{model_mapping,claude-haiku-4-5}', '"claude-sonnet-4-6"')
+WHERE platform = 'antigravity'
+  AND deleted_at IS NULL
+  AND credentials->'model_mapping'->>'claude-haiku-4-5' = 'claude-sonnet-4-5';
+
+-- 2. claude-haiku-4-5-20251001
+UPDATE accounts
+SET credentials = jsonb_set(credentials, '{model_mapping,claude-haiku-4-5-20251001}', '"claude-sonnet-4-6"')
+WHERE platform = 'antigravity'
+  AND deleted_at IS NULL
+  AND credentials->'model_mapping'->>'claude-haiku-4-5-20251001' = 'claude-sonnet-4-5';