fix(backend): 修复 CI 失败问题

修复内容：
1. 修复 6 个 golangci-lint 错误
   - 3 个 errcheck 错误：在 gateway_request_test.go 中添加类型断言检查
   - 3 个 gofmt 格式化问题：修复代码格式
2. 修复 API 契约测试失败
   - 在测试中添加缺失的字段：enable_identity_patch 和 identity_patch_prompt

所有测试和 linter 检查现已通过。

backend/internal/service/antigravity_gateway_service_test.go

@@ -17,14 +17,14 @@ func TestStripSignatureSensitiveBlocksFromClaudeRequest(t *testing.T) {
 		},
 		Messages: []antigravity.ClaudeMessage{
 			{
-				Role:    "assistant",
+				Role: "assistant",
 				Content: json.RawMessage(`[
 					{"type":"thinking","thinking":"secret plan","signature":""},
 					{"type":"tool_use","id":"t1","name":"Bash","input":{"command":"ls"}}
 				]`),
 			},
 			{
-				Role:    "user",
+				Role: "user",
 				Content: json.RawMessage(`[
 					{"type":"tool_result","tool_use_id":"t1","content":"ok","is_error":false},
 					{"type":"redacted_thinking","data":"..."}