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_irc.py

@@ -128,3 +128,17 @@ class TestFormat:
 
     def test_pong(self):
         assert format_msg("PONG", "server.example.com") == "PONG :server.example.com"
+
+    def test_trailing_starts_with_colon(self):
+        result = format_msg("PRIVMSG", "#ch", ":)")
+        assert result == "PRIVMSG #ch ::)"
+
+    def test_empty_trailing(self):
+        # Empty string has no space and no leading colon, but head exists
+        result = format_msg("PRIVMSG", "#ch", "")
+        assert result == "PRIVMSG #ch "
+
+    def test_multi_param_mode(self):
+        # No space in tail, not starting with colon, head exists -> no colon
+        result = format_msg("MODE", "#ch", "+o", "nick")
+        assert result == "MODE #ch +o nick"