fix(bot): don't substitute if there is no typo

This commit fixes an issue where a substitution command replays a
message because there was no needle to replace.

src/bot.zig

@@ -90,6 +90,9 @@ pub const Bot = struct {
                     command.author,
                     targets,
                 ) orelse return Error.NoMessage;
+                if (std.mem.count(u8, prev_msg.content, command.needle) == 0) {
+                    return Error.NoMessage;
+                }
                 const output = try std.mem.replaceOwned(
                     u8,
                     self.allocator,
@@ -255,7 +258,7 @@ test "hear wraps" {
     try std.testing.expectEqual(1024, bot.backlog.len());
 }
 
-test "execute_substitution_no_previous_message" {
+test "execute substitution no previous message" {
     var bot = try Bot.init(std.testing.allocator);
     defer bot.deinit();
     const cmd = UserCommand{ .substitute = .{
@@ -270,7 +273,7 @@ test "execute_substitution_no_previous_message" {
     ));
 }
 
-test "execute_substitution" {
+test "execute substitution" {
     var bot = try Bot.init(std.testing.allocator);
     defer bot.deinit();
 
@@ -298,3 +301,28 @@ test "execute_substitution" {
         else => unreachable,
     }
 }
+
+test "execute substitution with no matching needle" {
+    var bot = try Bot.init(std.testing.allocator);
+    defer bot.deinit();
+
+    // hear original message
+    const msg = try Message.init_owned(
+        std.testing.allocator,
+        1234,
+        "jassob",
+        "#test",
+        "original",
+    );
+    bot.hear(msg);
+
+    // execute substitution
+    const cmd = UserCommand{
+        .substitute = .{ .author = "jassob", .needle = "something else", .replacement = "weird" },
+    };
+    try std.testing.expectError(Error.NoMessage, bot.execute(
+        &cmd,
+        null,
+        "#test",
+    ));
+}