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/gateway_request.go

@@ -91,7 +91,7 @@ func FilterThinkingBlocks(body []byte) []byte {
 //   - Anthropic extended thinking has a structural constraint: when top-level `thinking` is enabled and the
 //     final message is an assistant prefill, the assistant content must start with a thinking block.
 //   - If we remove thinking blocks but keep top-level `thinking` enabled, we can trigger:
-//       "Expected `thinking` or `redacted_thinking`, but found `text`"
+//     "Expected `thinking` or `redacted_thinking`, but found `text`"
 //
 // Strategy (B: preserve content as text):
 //   - Disable top-level `thinking` (remove `thinking` field).