fix(discord): forum channel media + polish

Extend forum support from PR #10145:

- REST path (_send_discord): forum thread creation now uploads media
  files as multipart attachments on the starter message in a single
  call. Previously media files were silently dropped on the forum
  path.
- Websocket media paths (_send_file_attachment, send_voice, send_image,
  send_animation — covers send_image_file, send_video, send_document
  transitively): forum channels now go through a new _forum_post_file
  helper that creates a thread with the file as starter content,
  instead of failing via channel.send(file=...) which forums reject.
- _send_to_forum chunk follow-up failures are collected into
  raw_response['warnings'] so partial-send outcomes surface.
- Process-local probe cache (_DISCORD_CHANNEL_TYPE_PROBE_CACHE) avoids
  GET /channels/{id} on every uncached send after the first.
- Dedup of TestSendDiscordMedia that the PR merge-resolution left
  behind.
- Docs: Forum Channels section under website/docs/user-guide/messaging/discord.md.

Tests: 117 passed (22 new for forum+media, probe cache, warnings).

tests/gateway/test_discord_send.py

@@ -276,3 +276,113 @@ async def test_send_to_forum_create_thread_failure():
 
     assert result.success is False
     assert "rate limited" in result.error
+
+
+
+# ---------------------------------------------------------------------------
+# Forum follow-up chunk failure reporting + media on forum paths
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_send_to_forum_follow_up_chunk_failures_collected_as_warnings():
+    """Partial-send chunk failures surface in raw_response['warnings']."""
+    adapter = DiscordAdapter(PlatformConfig(enabled=True, token="***"))
+    adapter.MAX_MESSAGE_LENGTH = 20
+
+    chunk_msg_1 = SimpleNamespace(id=500)
+    # Every follow-up chunk fails — we should collect a warning per failure
+    thread_ch = SimpleNamespace(
+        id=555,
+        send=AsyncMock(side_effect=Exception("rate limited")),
+    )
+    thread = SimpleNamespace(id=555, message=chunk_msg_1, thread=thread_ch)
+    forum_channel = _discord_mod.ForumChannel()
+    forum_channel.id = 999
+    forum_channel.name = "ideas"
+    forum_channel.create_thread = AsyncMock(return_value=thread)
+    adapter._client = SimpleNamespace(
+        get_channel=lambda _chat_id: forum_channel,
+        fetch_channel=AsyncMock(),
+    )
+
+    # Long enough to produce multiple chunks
+    result = await adapter.send("999", "A" * 60)
+
+    # Starter message (first chunk) was delivered via create_thread, so send is
+    # successful overall — but follow-up chunks all failed and are reported.
+    assert result.success is True
+    assert result.message_id == "500"
+    warnings = (result.raw_response or {}).get("warnings") or []
+    assert len(warnings) >= 1
+    assert all("rate limited" in w for w in warnings)
+
+
+@pytest.mark.asyncio
+async def test_forum_post_file_creates_thread_with_attachment():
+    """_forum_post_file routes file-bearing sends to create_thread with file kwarg."""
+    adapter = DiscordAdapter(PlatformConfig(enabled=True, token="***"))
+
+    thread_ch = SimpleNamespace(id=777, send=AsyncMock())
+    thread = SimpleNamespace(id=777, message=SimpleNamespace(id=800), thread=thread_ch)
+    forum_channel = _discord_mod.ForumChannel()
+    forum_channel.id = 999
+    forum_channel.name = "ideas"
+    forum_channel.create_thread = AsyncMock(return_value=thread)
+
+    # discord.File is a real class; build a MagicMock that looks like one
+    fake_file = SimpleNamespace(filename="photo.png")
+
+    result = await adapter._forum_post_file(
+        forum_channel,
+        content="here is a photo",
+        file=fake_file,
+    )
+
+    assert result.success is True
+    assert result.message_id == "800"
+    forum_channel.create_thread.assert_awaited_once()
+    call_kwargs = forum_channel.create_thread.await_args.kwargs
+    assert call_kwargs["file"] is fake_file
+    assert call_kwargs["content"] == "here is a photo"
+    # Thread name derived from content's first line
+    assert call_kwargs["name"] == "here is a photo"
+
+
+@pytest.mark.asyncio
+async def test_forum_post_file_uses_filename_when_no_content():
+    """Thread name falls back to file.filename when no content is provided."""
+    adapter = DiscordAdapter(PlatformConfig(enabled=True, token="***"))
+
+    thread = SimpleNamespace(id=1, message=SimpleNamespace(id=2), thread=SimpleNamespace(id=1, send=AsyncMock()))
+    forum_channel = _discord_mod.ForumChannel()
+    forum_channel.id = 10
+    forum_channel.name = "forum"
+    forum_channel.create_thread = AsyncMock(return_value=thread)
+
+    fake_file = SimpleNamespace(filename="voice-message.ogg")
+    result = await adapter._forum_post_file(forum_channel, content="", file=fake_file)
+
+    assert result.success is True
+    call_kwargs = forum_channel.create_thread.await_args.kwargs
+    # Content was empty → thread name derived from filename
+    assert call_kwargs["name"] == "voice-message.ogg"
+
+
+@pytest.mark.asyncio
+async def test_forum_post_file_creation_failure():
+    """_forum_post_file returns a failed SendResult when create_thread raises."""
+    adapter = DiscordAdapter(PlatformConfig(enabled=True, token="***"))
+
+    forum_channel = _discord_mod.ForumChannel()
+    forum_channel.id = 999
+    forum_channel.create_thread = AsyncMock(side_effect=Exception("missing perms"))
+
+    result = await adapter._forum_post_file(
+        forum_channel,
+        content="hi",
+        file=SimpleNamespace(filename="x.png"),
+    )
+
+    assert result.success is False
+    assert "missing perms" in (result.error or "")