test: add channel filter and JSON log tests

TestChannelFilter: allowed/denied/PM/no-config/core-exempt/ampersand.
TestChannelConfig: TOML loading, defaults. TestJsonFormatter: fields,
exception, unicode, single-line, timestamp format.

tests/test_plugin.py

@@ -625,6 +625,58 @@ class TestBotAPI:
         assert bot.conn.sent == ["TOPIC #ch :new topic"]
 
 
+class TestChannelFilter:
+    """Test per-channel plugin allow/deny."""
+
+    @staticmethod
+    def _make_bot(channels_cfg: dict | None = None) -> Bot:
+        """Create a Bot with optional per-channel config."""
+        config = {
+            "server": {"host": "localhost", "port": 6667, "tls": False,
+                       "nick": "test", "user": "test", "realname": "test"},
+            "bot": {"prefix": "!", "channels": [], "plugins_dir": "plugins"},
+            "channels": channels_cfg or {},
+        }
+        return Bot(config, PluginRegistry())
+
+    def test_core_always_allowed(self):
+        bot = self._make_bot({"#locked": {"plugins": ["core"]}})
+        assert bot._plugin_allowed("core", "#locked") is True
+
+    def test_listed_plugin_allowed(self):
+        bot = self._make_bot({"#ops": {"plugins": ["core", "dns"]}})
+        assert bot._plugin_allowed("dns", "#ops") is True
+
+    def test_unlisted_plugin_denied(self):
+        bot = self._make_bot({"#ops": {"plugins": ["core", "dns"]}})
+        assert bot._plugin_allowed("encode", "#ops") is False
+
+    def test_unconfigured_channel_allows_all(self):
+        bot = self._make_bot({"#locked": {"plugins": ["core"]}})
+        assert bot._plugin_allowed("encode", "#open") is True
+
+    def test_no_channels_config_allows_all(self):
+        bot = self._make_bot()
+        assert bot._plugin_allowed("anything", "#test") is True
+
+    def test_pm_always_allowed(self):
+        bot = self._make_bot({"#locked": {"plugins": ["core"]}})
+        assert bot._plugin_allowed("encode", "someone") is True
+
+    def test_none_channel_allowed(self):
+        bot = self._make_bot({"#locked": {"plugins": ["core"]}})
+        assert bot._plugin_allowed("encode", None) is True
+
+    def test_channel_without_plugins_key(self):
+        bot = self._make_bot({"#other": {"some_setting": True}})
+        assert bot._plugin_allowed("encode", "#other") is True
+
+    def test_ampersand_channel(self):
+        bot = self._make_bot({"&local": {"plugins": ["core", "dns"]}})
+        assert bot._plugin_allowed("dns", "&local") is True
+        assert bot._plugin_allowed("encode", "&local") is False
+
+
 class TestSplitUtf8:
     """Test UTF-8 safe message splitting."""