refactor: extract ReadUpstreamResponseBody to deduplicate upstream response read + too-large error handling

Consolidates 9 call sites of resolveUpstreamResponseReadLimit + readUpstreamResponseBodyLimited + ErrUpstreamResponseBodyTooLarge error handling into a single ReadUpstreamResponseBody function with TooLargeWriter callback for API-format-specific error responses (Anthropic, OpenAI, countTokens).

backend/internal/service/upstream_response_limit_test.go

@@ -4,8 +4,10 @@ import (
 	"bytes"
 	"errors"
 	"testing"
+	"testing/iotest"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/require"
 )
 
@@ -35,3 +37,44 @@ func TestReadUpstreamResponseBodyLimited(t *testing.T) {
 		require.True(t, errors.Is(err, ErrUpstreamResponseBodyTooLarge))
 	})
 }
+
+func TestReadUpstreamResponseBody(t *testing.T) {
+	t.Run("within limit", func(t *testing.T) {
+		body, err := ReadUpstreamResponseBody(bytes.NewReader([]byte("ok")), nil, nil, nil)
+		require.NoError(t, err)
+		require.Equal(t, []byte("ok"), body)
+	})
+
+	t.Run("exceeds limit calls onTooLarge", func(t *testing.T) {
+		cfg := &config.Config{}
+		cfg.Gateway.UpstreamResponseReadMaxBytes = 3
+
+		called := false
+		onTooLarge := func(_ *gin.Context) { called = true }
+
+		body, err := ReadUpstreamResponseBody(bytes.NewReader([]byte("toolong")), cfg, nil, onTooLarge)
+		require.Nil(t, body)
+		require.True(t, errors.Is(err, ErrUpstreamResponseBodyTooLarge))
+		require.True(t, called)
+	})
+
+	t.Run("nil onTooLarge does not panic", func(t *testing.T) {
+		cfg := &config.Config{}
+		cfg.Gateway.UpstreamResponseReadMaxBytes = 3
+
+		body, err := ReadUpstreamResponseBody(bytes.NewReader([]byte("toolong")), cfg, nil, nil)
+		require.Nil(t, body)
+		require.True(t, errors.Is(err, ErrUpstreamResponseBodyTooLarge))
+	})
+
+	t.Run("io error does not call onTooLarge", func(t *testing.T) {
+		called := false
+		onTooLarge := func(_ *gin.Context) { called = true }
+
+		body, err := ReadUpstreamResponseBody(iotest.ErrReader(errors.New("disk failure")), nil, nil, onTooLarge)
+		require.Nil(t, body)
+		require.Error(t, err)
+		require.False(t, errors.Is(err, ErrUpstreamResponseBodyTooLarge))
+		require.False(t, called)
+	})
+}