fix(discord): properly route slash event handling in threads

Cherry-picked from PR #2017 by @simpolism. Fixes #2011.

Discord slash commands in threads were missing thread_id in the
SessionSource, causing them to route to the parent channel session.
Commands like /usage and /reset returned wrong data or affected the
wrong session.

Detects discord.Thread channels in _build_slash_event and sets
chat_type='thread' with thread_id. Two tests added.

tests/gateway/test_discord_slash_commands.py

@@ -241,6 +241,42 @@ async def test_dispatch_thread_session_builds_thread_event(adapter):
     assert "TestGuild" in event.source.chat_name
 
 
+# ------------------------------------------------------------------
+# _build_slash_event — preserve thread context for native slash commands
+# ------------------------------------------------------------------
+
+
+def test_build_slash_event_preserves_thread_context(adapter):
+    interaction = SimpleNamespace(
+        channel=_FakeThreadChannel(channel_id=555, name="Planning"),
+        channel_id=555,
+        user=SimpleNamespace(display_name="Jezza", id=42),
+    )
+
+    event = adapter._build_slash_event(interaction, "/status")
+
+    assert event.text == "/status"
+    assert event.source.chat_id == "555"
+    assert event.source.chat_type == "thread"
+    assert event.source.thread_id == "555"
+    assert "TestGuild" in event.source.chat_name
+
+
+def test_build_slash_event_uses_group_context_for_channels(adapter):
+    interaction = SimpleNamespace(
+        channel=_FakeTextChannel(channel_id=123, name="general"),
+        channel_id=123,
+        user=SimpleNamespace(display_name="Jezza", id=42),
+    )
+
+    event = adapter._build_slash_event(interaction, "/status")
+
+    assert event.source.chat_id == "123"
+    assert event.source.chat_type == "group"
+    assert event.source.thread_id is None
+    assert "TestGuild / #general" == event.source.chat_name
+
+
 # ------------------------------------------------------------------
 # Auto-thread: _auto_create_thread
 # ------------------------------------------------------------------