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_config.py

@@ -0,0 +1,93 @@
+"""Tests for configuration loading and merging."""
+
+import copy
+from pathlib import Path
+
+import pytest
+
+from derp.config import DEFAULTS, _merge, load, resolve_config
+
+
+class TestMerge:
+    """Test deep merge logic."""
+
+    def test_flat_override(self):
+        base = {"a": 1, "b": 2}
+        result = _merge(base, {"b": 99})
+        assert result == {"a": 1, "b": 99}
+
+    def test_nested_merge(self):
+        base = {"server": {"host": "a", "port": 1}}
+        result = _merge(base, {"server": {"port": 2}})
+        assert result == {"server": {"host": "a", "port": 2}}
+
+    def test_list_replacement(self):
+        base = {"channels": ["#old"]}
+        result = _merge(base, {"channels": ["#new"]})
+        assert result == {"channels": ["#new"]}
+
+    def test_empty_override(self):
+        base = {"a": 1, "b": 2}
+        result = _merge(base, {})
+        assert result == {"a": 1, "b": 2}
+
+    def test_new_keys(self):
+        base = {"a": 1}
+        result = _merge(base, {"b": 2})
+        assert result == {"a": 1, "b": 2}
+
+    def test_base_not_mutated(self):
+        base = {"server": {"host": "original"}}
+        original = copy.deepcopy(base)
+        _merge(base, {"server": {"host": "changed"}})
+        assert base == original
+
+
+class TestLoad:
+    """Test TOML file loading."""
+
+    def test_minimal_toml(self, tmp_path: Path):
+        config_file = tmp_path / "test.toml"
+        config_file.write_text('[server]\nnick = "testbot"\n')
+        result = load(config_file)
+        # Overridden value
+        assert result["server"]["nick"] == "testbot"
+        # Default preserved
+        assert result["server"]["host"] == "irc.libera.chat"
+        assert result["server"]["port"] == 6697
+
+    def test_full_override(self, tmp_path: Path):
+        config_file = tmp_path / "test.toml"
+        config_file.write_text(
+            '[server]\nhost = "custom.irc"\nport = 6667\n'
+            '[bot]\nprefix = "."\n'
+        )
+        result = load(config_file)
+        assert result["server"]["host"] == "custom.irc"
+        assert result["server"]["port"] == 6667
+        assert result["bot"]["prefix"] == "."
+        # Unset defaults still present
+        assert result["bot"]["channels"] == ["#test"]
+
+
+class TestResolveConfig:
+    """Test config path resolution and fallback."""
+
+    def test_explicit_path(self, tmp_path: Path):
+        config_file = tmp_path / "explicit.toml"
+        config_file.write_text('[server]\nnick = "explicit"\n')
+        result = resolve_config(str(config_file))
+        assert result["server"]["nick"] == "explicit"
+
+    def test_no_file_fallback(self, tmp_path: Path, monkeypatch: "pytest.MonkeyPatch"):
+        # Run from tmp_path so config/derp.toml isn't found
+        monkeypatch.chdir(tmp_path)
+        result = resolve_config(str(tmp_path / "nonexistent.toml"))
+        # Should fall back to DEFAULTS
+        assert result["server"]["host"] == DEFAULTS["server"]["host"]
+        assert result["bot"]["prefix"] == DEFAULTS["bot"]["prefix"]
+
+    def test_defaults_not_mutated(self):
+        original = copy.deepcopy(DEFAULTS)
+        resolve_config(None)
+        assert DEFAULTS == original