test: add config, format_msg, and Bot API tests

New test_config.py: merge, load, resolve_config tests.
Extend test_irc.py: format_msg edge cases (colon, empty, multi-param).
Extend test_plugin.py: Bot API via FakeConnection, _split_utf8 tests.
Test count: 92 -> 120.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_plugin.py

@@ -1,9 +1,10 @@
 """Tests for the plugin system."""
 
+import asyncio
 import textwrap
 from pathlib import Path
 
-from derp.bot import _AMBIGUOUS, Bot
+from derp.bot import _AMBIGUOUS, Bot, _split_utf8
 from derp.irc import Message
 from derp.plugin import PluginRegistry, command, event
 from derp.state import StateStore
@@ -545,3 +546,110 @@ class TestStateStore:
         store.close()
         store2 = StateStore(db_path)
         assert store2.get("plug", "persist") == "yes"
+
+
+class _FakeConnection:
+    """Minimal IRCConnection stand-in that captures sent lines."""
+
+    def __init__(self):
+        self.sent: list[str] = []
+
+    async def send(self, line: str) -> None:
+        self.sent.append(line)
+
+
+def _make_test_bot() -> Bot:
+    """Create a Bot with a FakeConnection for testing the public API."""
+    config = {
+        "server": {"host": "localhost", "port": 6667, "tls": False,
+                    "nick": "test", "user": "test", "realname": "test"},
+        "bot": {"prefix": "!", "channels": [], "plugins_dir": "plugins"},
+    }
+    bot = Bot(config, PluginRegistry())
+    bot.conn = _FakeConnection()  # type: ignore[assignment]
+    return bot
+
+
+class TestBotAPI:
+    """Test Bot public API methods via FakeConnection."""
+
+    def test_send(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.send("#ch", "hello world"))
+        assert bot.conn.sent == ["PRIVMSG #ch :hello world"]
+
+    def test_send_single_word(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.send("#ch", "hello"))
+        assert bot.conn.sent == ["PRIVMSG #ch hello"]
+
+    def test_send_multiline(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.send("#ch", "line one\nline two"))
+        assert len(bot.conn.sent) == 2
+        assert bot.conn.sent[0] == "PRIVMSG #ch :line one"
+        assert bot.conn.sent[1] == "PRIVMSG #ch :line two"
+
+    def test_reply_channel(self):
+        bot = _make_test_bot()
+        msg = Message(raw="", prefix="nick!u@h", nick="nick",
+                      command="PRIVMSG", params=["#ch", "hi"], tags={})
+        asyncio.run(bot.reply(msg, "hello there"))
+        assert bot.conn.sent == ["PRIVMSG #ch :hello there"]
+
+    def test_reply_pm(self):
+        bot = _make_test_bot()
+        msg = Message(raw="", prefix="nick!u@h", nick="nick",
+                      command="PRIVMSG", params=["testbot", "hi"], tags={})
+        asyncio.run(bot.reply(msg, "hello there"))
+        assert bot.conn.sent == ["PRIVMSG nick :hello there"]
+
+    def test_join(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.join("#test"))
+        assert bot.conn.sent == ["JOIN :#test"]
+
+    def test_kick(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.kick("#ch", "baduser", "bad behavior"))
+        assert bot.conn.sent == ["KICK #ch baduser :bad behavior"]
+
+    def test_mode(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.mode("#ch", "+o", "nick"))
+        assert bot.conn.sent == ["MODE #ch +o nick"]
+
+    def test_set_topic(self):
+        bot = _make_test_bot()
+        asyncio.run(bot.set_topic("#ch", "new topic"))
+        assert bot.conn.sent == ["TOPIC #ch :new topic"]
+
+
+class TestSplitUtf8:
+    """Test UTF-8 safe message splitting."""
+
+    def test_short_text(self):
+        assert _split_utf8("hello", 100) == ["hello"]
+
+    def test_ascii_split(self):
+        text = "a" * 20
+        chunks = _split_utf8(text, 10)
+        assert len(chunks) == 2
+        assert all(len(c.encode("utf-8")) <= 10 for c in chunks)
+        assert "".join(chunks) == text
+
+    def test_multibyte_boundary(self):
+        # Each char is 3 bytes in UTF-8
+        text = "\u00e9" * 10  # e-acute, 2 bytes each -> 20 bytes total
+        chunks = _split_utf8(text, 7)
+        recombined = "".join(chunks)
+        assert recombined == text
+        for chunk in chunks:
+            assert len(chunk.encode("utf-8")) <= 7
+
+    def test_empty(self):
+        assert _split_utf8("", 10) == [""]
+
+    def test_exact_fit(self):
+        text = "abcde"
+        assert _split_utf8(text, 5) == ["abcde"]