fix(discord): honor wildcard '*' in DISCORD_ALLOWED_CHANNELS

allowed_channels: "*" in config (or DISCORD_ALLOWED_CHANNELS="*" env var)
is meant to allow all channels, but the check was comparing numeric channel
IDs against the literal string set {"*"} via set intersection — always empty,
so every message was silently dropped.

Add a "*" short-circuit before the set intersection, consistent with every
other platform's allowlist handling (Signal, Slack, Telegram all do this).

Fixes #14920

tests/gateway/test_discord_allowed_channels.py

@@ -0,0 +1,48 @@
+"""Regression guard for #14920: DISCORD_ALLOWED_CHANNELS="*" should allow all channels.
+
+Setting allowed_channels: "*" in config (or DISCORD_ALLOWED_CHANNELS="*" as env var)
+must behave as a wildcard — i.e. the bot responds in every channel. Previously the
+literal string "*" was placed into the set and compared against numeric channel IDs via
+set-intersection, which always produced an empty set, causing every message to be
+silently dropped.
+"""
+
+import unittest
+
+
+def _channel_is_allowed(channel_id: str, allowed_channels_raw: str) -> bool:
+    """Replicate the channel-allow-list check from discord.py on_message."""
+    if not allowed_channels_raw:
+        return True
+    allowed_channels = {ch.strip() for ch in allowed_channels_raw.split(",") if ch.strip()}
+    if "*" in allowed_channels:
+        return True
+    return bool({channel_id} & allowed_channels)
+
+
+class TestDiscordAllowedChannelsWildcard(unittest.TestCase):
+    """Wildcard and channel-list behaviour for DISCORD_ALLOWED_CHANNELS."""
+
+    def test_wildcard_allows_any_channel(self):
+        """'*' should allow messages from any channel ID."""
+        self.assertTrue(_channel_is_allowed("1234567890", "*"))
+
+    def test_wildcard_in_list_allows_any_channel(self):
+        """'*' mixed with other entries still allows any channel."""
+        self.assertTrue(_channel_is_allowed("9999999999", "111,*,222"))
+
+    def test_exact_match_allowed(self):
+        """Channel ID present in the explicit list is allowed."""
+        self.assertTrue(_channel_is_allowed("1234567890", "1234567890,9876543210"))
+
+    def test_non_matching_channel_blocked(self):
+        """Channel ID absent from the explicit list is blocked."""
+        self.assertFalse(_channel_is_allowed("5555555555", "1234567890,9876543210"))
+
+    def test_empty_allowlist_allows_all(self):
+        """Empty DISCORD_ALLOWED_CHANNELS means no restriction."""
+        self.assertTrue(_channel_is_allowed("1234567890", ""))
+
+    def test_whitespace_only_entry_ignored(self):
+        """Entries that are only whitespace are stripped and ignored."""
+        self.assertFalse(_channel_is_allowed("1234567890", "  ,  "))