Refuse a PRIVMSG or NOTICE whose body or meta has the wrong type (closes #245)

Over the HTTP API, a PRIVMSG or NOTICE body was checked only by
decoding it into a list of strings. JSON null decodes into an empty
string there, and meta was not checked at all, so a null line, or a
meta that is not an object, reached every recipient.

handlePrivmsg now answers 400 invalid request body, before anything
else, when body is present and is not an array of strings, or meta is
present and is not an object. A missing or empty body still gets
ERR_NOTEXTTOSEND. README.md says so.

Model: opus-5-5

internal/handlers/api.go

@@ -1141,6 +1141,16 @@ func (hdlr *Handlers) handlePrivmsg(
 	meta json.RawMessage,
 	bodyLines func() []string,
 ) {
+	if !validBodyAndMeta(body, meta) {
+		hdlr.respondError(
+			writer, request,
+			"invalid request body",
+			http.StatusBadRequest,
+		)
+
+		return
+	}
+
 	if target == "" {
 		hdlr.enqueueNumeric(
 			request.Context(), clientID,
@@ -1189,6 +1199,40 @@ func (hdlr *Handlers) handlePrivmsg(
 	)
 }
 
+// validBodyAndMeta reports whether body, when present, is an
+// array of strings and meta, when present, is an object.
+// Decoding into []string would turn a null line into "", so
+// each line is decoded as any and checked. A null body or
+// meta decodes without an error into nil.
+func validBodyAndMeta(body, meta json.RawMessage) bool {
+	if body != nil {
+		var lines []any
+
+		err := json.Unmarshal(body, &lines)
+		if err != nil || lines == nil {
+			return false
+		}
+
+		for _, line := range lines {
+			_, isString := line.(string)
+			if !isString {
+				return false
+			}
+		}
+	}
+
+	if meta != nil {
+		var fields map[string]any
+
+		err := json.Unmarshal(meta, &fields)
+		if err != nil || fields == nil {
+			return false
+		}
+	}
+
+	return true
+}
+
 // respondIRCError enqueues a numeric error reply, notifies
 // the broker, and sends HTTP 200 with {"status":"error"}.
 func (hdlr *Handlers) respondIRCError(