A SimpleX Chat bot that answers arithmetic (closes #1)

Remove the template's HTTP service, database and fx wiring. Add exact
arithmetic on go/parser and go/constant, a client that runs simplex-chat
as a child process and drives its WebSocket API, and the bot, which keeps
an auto-accepting address and replies to each message. The image adds the
checksum-pinned simplex-chat v7.0.2 on Ubuntu 22.04.

Model: opus-5-5

internal/bot/bot_test.go

@@ -0,0 +1,35 @@
+package bot_test
+
+import (
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/simplexcalc/internal/bot"
+	"sneak.berlin/go/simplexcalc/internal/calc"
+)
+
+// TestReply: a result is sent bare, and every way of failing gets its
+// own short explanation rather than silence.
+func TestReply(t *testing.T) {
+	t.Parallel()
+
+	for in, want := range map[string]string{
+		"2 + 2":   "4",
+		"5 * 5/2": "12.5",
+	} {
+		if got := bot.Reply(in); got != want {
+			t.Errorf("Reply(%q) = %q, want %q", in, got, want)
+		}
+	}
+
+	for in, want := range map[string]string{
+		"hello": "I only understand arithmetic",
+		"1 / 0": "I cannot divide by zero.",
+		"1e400": "The result is too large for me.",
+		strings.Repeat("1+", calc.MaxInputLength) + "1": "That is too long for me",
+	} {
+		if got := bot.Reply(in); !strings.HasPrefix(got, want) {
+			t.Errorf("Reply(%q) = %q, want it to start %q", in, got, want)
+		}
+	}
+}