Return sink write errors from every handler (closes #22)

The console and JSON handlers threw away the error from their write to
stdout, so a lost log line looked delivered. They now return it,
wrapped. The webhook handler also returns an error for a status outside
2xx. The multiplex handler passes the record to every handler and
returns their errors joined with errors.Join instead of stopping at the
first.

Both stdout handlers gain an unexported writer, nil meaning os.Stdout at
write time, so tests can supply a sink that fails. The README says what
each handler returns and that the slog.Logger methods discard a
handler's error.

Model: opus-5-5

json_handler.go

@@ -4,12 +4,16 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"log/slog"
 	"os"
 )
 
 // JSONHandler writes each log record to stdout as a JSON document.
 type JSONHandler struct {
+	// out is where records are written. Nil means os.Stdout, looked up on
+	// every write so that a reassigned os.Stdout is followed.
+	out   io.Writer
 	attrs handlerAttrs
 }
 
@@ -19,14 +23,22 @@ func NewJSONHandler() *JSONHandler {
 }
 
 // Handle marshals the record, with its attributes, to one JSON object and
-// writes it to stdout.
+// writes it to stdout. A failed write is returned as an error.
 func (j *JSONHandler) Handle(_ context.Context, record slog.Record) error {
 	jsonData, err := json.Marshal(recordToMap(record, j.attrs))
 	if err != nil {
 		return err
 	}
 
-	_, _ = fmt.Fprintln(os.Stdout, string(jsonData))
+	out := j.out
+	if out == nil {
+		out = os.Stdout
+	}
+
+	_, err = fmt.Fprintln(out, string(jsonData))
+	if err != nil {
+		return fmt.Errorf("error writing log record: %w", err)
+	}
 
 	return nil
 }
@@ -44,7 +56,7 @@ func (j *JSONHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
 		return j
 	}
 
-	return &JSONHandler{attrs: j.attrs.withAttrs(attrs)}
+	return &JSONHandler{out: j.out, attrs: j.attrs.withAttrs(attrs)}
 }
 
 // WithGroup returns a new handler that nests later attributes in an
@@ -54,5 +66,5 @@ func (j *JSONHandler) WithGroup(name string) slog.Handler {
 		return j
 	}
 
-	return &JSONHandler{attrs: j.attrs.withGroup(name)}
+	return &JSONHandler{out: j.out, attrs: j.attrs.withGroup(name)}
 }