Apply MAX_MESSAGE_SIZE to the whole request body (closes #250)

Creating a session, logging in and posting a message stopped reading
after the first JSON value, so the limit never saw the rest of the
body and trailing bytes were ignored. All three now read the whole
limited body and decode it with json.Unmarshal. A body over the limit,
or anything but whitespace after the value, gets 400 invalid request
body.

Model: opus-5-5

internal/handlers/handlers.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"io"
 	"log/slog"
 	"net/http"
 	"time"
@@ -159,6 +160,18 @@ func (hdlr *Handlers) respondError(
 	)
 }
 
+// decodeJSON reads the whole request body and decodes it into dest.
+// It fails if the body is larger than the http.MaxBytesReader limit,
+// or holds anything other than one JSON value and whitespace.
+func decodeJSON(request *http.Request, dest any) error {
+	body, err := io.ReadAll(request.Body)
+	if err != nil {
+		return err
+	}
+
+	return json.Unmarshal(body, dest)
+}
+
 func (hdlr *Handlers) idleTimeout() time.Duration {
 	raw := hdlr.params.Config.SessionIdleTimeout
 	if raw == "" {