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_test.go

@@ -8,10 +8,12 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"maps"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
 	"os"
+	"reflect"
 	"slices"
 	"strconv"
 	"strings"
@@ -1056,6 +1058,132 @@ func TestMessageMissingTo(t *testing.T) {
 	}
 }
 
+// TestMessageInvalidBodyOrMeta checks that a PRIVMSG or
+// NOTICE, to a channel or a nick, whose body is not an array
+// of strings or whose meta is not an object gets 400 and
+// reaches nobody.
+func TestMessageInvalidBodyOrMeta(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name   string
+		fields map[string]any
+	}{
+		{"null line", map[string]any{bodyKey: []any{"hi", nil}}},
+		{"number line", map[string]any{bodyKey: []any{"hi", 1}}},
+		{"object line", map[string]any{bodyKey: []any{map[string]any{}}}},
+		{"null body", map[string]any{bodyKey: nil}},
+		{"object body", map[string]any{bodyKey: map[string]any{}}},
+		{"null meta", map[string]any{bodyKey: []string{"hi"}, metaKey: nil}},
+		{"array meta", map[string]any{bodyKey: []string{"hi"}, metaKey: []string{}}},
+		{"string meta", map[string]any{bodyKey: []string{"hi"}, metaKey: "hi"}},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			const channel = "#badmsg"
+
+			tserver := newTestServer(t)
+			aliceToken := tserver.createSession("alice_badmsg")
+			bobToken := tserver.createSession("bob_badmsg")
+
+			tserver.sendCommand(aliceToken, map[string]any{
+				commandKey: joinCmd, toKey: channel,
+			})
+			tserver.sendCommand(bobToken, map[string]any{
+				commandKey: joinCmd, toKey: channel,
+			})
+
+			_, bobLastID := tserver.pollMessages(bobToken, 0)
+
+			for _, command := range []string{privmsgCmd, irc.CmdNotice} {
+				for _, target := range []string{channel, "bob_badmsg"} {
+					request := map[string]any{
+						commandKey: command, toKey: target,
+					}
+					maps.Copy(request, tc.fields)
+
+					status, result := tserver.sendCommand(
+						aliceToken, request,
+					)
+					if status != http.StatusBadRequest ||
+						result["error"] != "invalid request body" {
+						t.Errorf(
+							"%s to %s: expected 400 invalid "+
+								"request body, got %d: %v",
+							command, target, status, result,
+						)
+					}
+				}
+			}
+
+			msgs, _ := tserver.pollMessages(bobToken, bobLastID)
+			if len(msgs) != 0 {
+				t.Fatalf("expected no messages, bob got %v", msgs)
+			}
+		})
+	}
+}
+
+// TestMessageRelaysBodyAndMeta checks that a PRIVMSG or
+// NOTICE, to a channel or a nick, reaches its recipient with
+// body and meta as sent.
+func TestMessageRelaysBodyAndMeta(t *testing.T) {
+	t.Parallel()
+
+	const channel = "#relaymsg"
+
+	tserver := newTestServer(t)
+	aliceToken := tserver.createSession("alice_relaymsg")
+	bobToken := tserver.createSession("bob_relaymsg")
+
+	tserver.sendCommand(aliceToken, map[string]any{
+		commandKey: joinCmd, toKey: channel,
+	})
+	tserver.sendCommand(bobToken, map[string]any{
+		commandKey: joinCmd, toKey: channel,
+	})
+
+	_, bobLastID := tserver.pollMessages(bobToken, 0)
+
+	body := []any{"line one", ""}
+	meta := map[string]any{"sig": "abc", "alg": "ed25519"}
+
+	for _, command := range []string{privmsgCmd, irc.CmdNotice} {
+		for _, target := range []string{channel, "bob_relaymsg"} {
+			status, result := tserver.sendCommand(
+				aliceToken,
+				map[string]any{
+					commandKey: command, toKey: target,
+					bodyKey: body, metaKey: meta,
+				},
+			)
+			if status != http.StatusOK {
+				t.Fatalf(
+					"%s to %s: expected 200, got %d: %v",
+					command, target, status, result,
+				)
+			}
+
+			var msgs []map[string]any
+
+			msgs, bobLastID = tserver.pollMessages(
+				bobToken, bobLastID,
+			)
+			if len(msgs) != 1 ||
+				msgs[0][commandKey] != command ||
+				!reflect.DeepEqual(msgs[0][bodyKey], body) ||
+				!reflect.DeepEqual(msgs[0][metaKey], meta) {
+				t.Fatalf(
+					"%s to %s: bob got %v", command, target, msgs,
+				)
+			}
+		}
+	}
+}
+
 func TestNonMemberCannotSend(t *testing.T) {
 	t.Parallel()