fix(provider): 统一响应体超时错误分类

将收到响应头后在读取 JSON 或 Veo 视频响应体阶段发生的 deadline/网络超时统一归类为 terminal timeout，保留 HTTP 状态、request ID 和 wire 证据，避免继续返回 response_read_error。

验证：go test ./... -count=1；go test ./internal/clients ./internal/runner -count=1；go vet ./internal/clients ./internal/runner；gofmt。

apps/api/internal/clients/media_timeout_test.go

@@ -2,10 +2,26 @@ package clients
 
 import (
 	"context"
+	"io"
+	"net/http"
 	"testing"
 	"time"
 )
 
+type timeoutResponseBody struct {
+	read bool
+}
+
+func (body *timeoutResponseBody) Read(buffer []byte) (int, error) {
+	if body.read {
+		return 0, context.DeadlineExceeded
+	}
+	body.read = true
+	return copy(buffer, []byte(`{"partial":`)), nil
+}
+
+func (*timeoutResponseBody) Close() error { return nil }
+
 func TestProviderRequestTimeoutUsesMediaDefaults(t *testing.T) {
 	for _, test := range []struct {
 		kind string
@@ -29,6 +45,25 @@ func TestTransportTimeoutUsesTerminalTimeoutCode(t *testing.T) {
 	}
 }
 
+func TestResponseBodyTimeoutUsesTerminalTimeoutCode(t *testing.T) {
+	response := &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     make(http.Header),
+		Body:       io.NopCloser(&timeoutResponseBody{}),
+	}
+	_, wire, err := decodeHTTPResponseForProtocol(response, ProtocolOpenAIImages)
+	if err == nil {
+		t.Fatal("expected response body timeout")
+	}
+	clientErr, ok := err.(*ClientError)
+	if !ok || clientErr.Code != "timeout" || clientErr.Retryable {
+		t.Fatalf("response body timeout classification = %#v, want terminal timeout", err)
+	}
+	if clientErr.StatusCode != http.StatusOK || clientErr.Wire != wire {
+		t.Fatalf("response body timeout lost response metadata: %#v", clientErr)
+	}
+}
+
 func TestMediaPollTimeoutsUseTaskDefaultsWithoutPlatformOverride(t *testing.T) {
 	image := Request{Kind: "images.generations", Candidate: storeCandidateWithConfig("", "", nil, nil)}
 	video := Request{Kind: "videos.generations", Candidate: storeCandidateWithConfig("", "", nil, nil)}