translator: handle numeric OpenAI error.code in Anthropic→OpenAI error translation (envoyproxy#2161)

**Description**

Anthropic→OpenAI error translation could fail with an internal 500 when
an OpenAI-compatible upstream returned a structured JSON error where
`error.code` was numeric (e.g. `400`) instead of string. This change
makes the error model tolerant to both representations so upstream 4xx
diagnostics can still be translated and surfaced to Anthropic clients.

- **Schema decoding hardening (`internal/apischema/openai/openai.go`)**
  - Added `UnmarshalJSON` for `openai.ErrorType`.
  - `error.code` now accepts:
    - JSON string (`"code":"400"`)
    - JSON number (`"code":400`)
    - `null` / omitted
- Numeric values are normalized to the existing internal `*string`
field.

- **Translator regression coverage
(`internal/translator/anthropic_openai_test.go`)**
- Extended `TestAnthropicToOpenAITranslator_ResponseError` with a JSON
error fixture containing numeric `code`.
- Confirms response still maps to Anthropic error envelope with expected
`type` and `message`.

```json
{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "Bad request",
    "param": null,
    "code": 400
  }
}
```

**Related Issues/PRs (if applicable)**

Fixes envoyproxy#2151

**Special notes for reviewers (if applicable)**

N/A

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Ignasi Barrera <ignasi@tetrate.io>
Signed-off-by: yxia216 <yxia216@bloomberg.net>

Author: 2 people

internal/apischema/openai/openai.go

@@ -1662,6 +1662,47 @@ type ErrorType struct {
 	EventID *string `json:"event_id,omitempty"`
 }
 
+// UnmarshalJSON allows OpenAI-compatible backends to provide `error.code` as either a JSON string or number.
+func (e *ErrorType) UnmarshalJSON(data []byte) error {
+	type errorTypeAlias ErrorType
+	aux := struct {
+		errorTypeAlias
+		Code json.RawMessage `json:"code,omitempty"`
+	}{}
+
+	if err := json.Unmarshal(data, &aux); err != nil {
+		return err
+	}
+
+	*e = ErrorType(aux.errorTypeAlias)
+	if len(aux.Code) == 0 || string(aux.Code) == "null" {
+		e.Code = nil
+		return nil
+	}
+
+	var code string
+	if err := json.Unmarshal(aux.Code, &code); err == nil {
+		e.Code = &code
+		return nil
+	}
+
+	var codeInt int64
+	if err := json.Unmarshal(aux.Code, &codeInt); err == nil {
+		code = strconv.FormatInt(codeInt, 10)
+		e.Code = &code
+		return nil
+	}
+
+	var codeFloat float64
+	if err := json.Unmarshal(aux.Code, &codeFloat); err == nil {
+		code = strconv.FormatFloat(codeFloat, 'f', -1, 64)
+		e.Code = &code
+		return nil
+	}
+
+	return fmt.Errorf("error.code must be string or number")
+}
+
 // ModelList is described in the OpenAI API documentation
 // https://platform.openai.com/docs/api-reference/models/list
 type ModelList struct {

internal/translator/anthropic_openai_test.go

@@ -430,6 +430,13 @@ func TestAnthropicToOpenAITranslator_ResponseError(t *testing.T) {
 			wantErrType: "invalid_request_error",
 			wantErrMsg:  "Bad request",
 		},
+		{
+			name:        "JSON error with numeric code from OpenAI-compatible backend",
+			headers:     map[string]string{contentTypeHeaderName: "application/json"},
+			body:        `{"type":"error","error":{"type":"invalid_request_error","message":"Bad request","param":null,"code":400}}`,
+			wantErrType: "invalid_request_error",
+			wantErrMsg:  "Bad request",
+		},
 		{
 			name:        "non-JSON 400 error",
 			headers:     map[string]string{statusHeaderName: "400", contentTypeHeaderName: "text/plain"},