fix(sync): tolerate list inputs for Discord server_ids / channels (#15790) (#15809)

## Summary

Fixes #15790.

Every Discord sync launched from the current Web UI crashes immediately
with:

```
'list' object has no attribute 'split'
```

The error is raised in
[rag/svr/sync_data_source.py:650-651](rag/svr/sync_data_source.py#L650-L651):

```python
server_ids=server_ids.split(",") if server_ids else [],
channel_names=channel_names.split(",") if channel_names else [],
```

### Root cause

Three independent bugs stack here, all in the Discord branch of
`sync_data_source.py`:

1. **Type mismatch (the user's exact error).** The current form at
[web/src/pages/user-setting/data-source/constant/index.tsx:833-843](web/src/pages/user-setting/data-source/constant/index.tsx#L833-L843)
uses `FormFieldType.Tag` for both **Server IDs** and **Channels**:

    ```tsx
{ label: 'Server IDs', name: 'config.server_ids', type:
FormFieldType.Tag, required: false },
{ label: 'Channels', name: 'config.channels', type: FormFieldType.Tag,
required: false },
    ```

Tag inputs serialise to **lists**, not comma-separated strings. The
backend `.split(",")` then explodes on the very first sync.

2. **Field-name mismatch.** The form writes `config.channels`. The
backend reads `self.conf.get("channel_names", None)`. Even if
`.split(",")` were fixed, channels would silently be empty for every
UI-created source.

3. **Int conversion missing.**
[common/data_source/discord_connector.py:82](common/data_source/discord_connector.py#L82)
types `server_ids` as `list[int]` (Discord guild IDs are integers); the
previous `.split(",")` produced strings, so the `channel.guild.id not in
server_ids` filter at
[discord_connector.py:92](common/data_source/discord_connector.py#L92)
silently never matched.

So even the configurations that didn't crash were also broken — there is
no path through the current code that actually filtered by server id
from a UI-created source.

### Fix

A 39-line patch in one function:

- New `Discord._coerce_str_list` static method: accepts `None` / `""` /
`list` / `tuple` / `set` / scalar / comma-separated str, returns a clean
`list[str]` with whitespace trimmed and empty entries dropped.
Smoke-tested against the 10 input shapes that can hit it (see Test
plan).
- `_generate` reads `config.channels` first (the form's actual key) and
falls back to `config.channel_names`, so SDK callers and legacy configs
that already shipped with the old key keep working.
- `server_ids` is coerced to `list[int]`. Non-integer entries are logged
and dropped instead of crashing the sync, so a single malformed tag from
the form doesn't tank the rest of the run.

### What this PR does NOT change

- **Web form key (`config.channels`)** — kept as-is. Renaming it to
`channel_names` would force a UI migration and break in-flight configs;
the backend fallback solves the same problem more safely.
- **`common/data_source/discord_connector.py`** — its signature was
already correct.
- **Other connectors (Slack, Gmail, Confluence, etc.)** — they don't
crash today and were not in the issue's scope.

## Test plan

`Discord._coerce_str_list` has been exercised against all ten realistic
input shapes — list, tuple, set, comma-separated string, str with extra
whitespace, empty entries, integers from a Tag input, None, empty list,
single trailing comma. All pass.

rag/svr/sync_data_source.py

@@ -641,14 +641,61 @@ class Notion(SyncBase):
 class Discord(SyncBase):
     SOURCE_NAME: str = FileSource.DISCORD
 
+    @staticmethod
+    def _coerce_str_list(raw):
+        """Normalise a config field that may arrive as a list (Tag input from
+        the new web form), a comma-separated string (legacy/SDK callers), or
+        None into a clean ``list[str]`` with empty entries dropped.
+
+        Fixes #15790 — the previous ``.split(',')`` call assumed a string and
+        raised ``'list' object has no attribute 'split'`` for any config saved
+        through the current UI.
+        """
+        if not raw:
+            return []
+        if isinstance(raw, str):
+            items = raw.split(",")
+        elif isinstance(raw, (list, tuple, set)):
+            items = list(raw)
+        else:
+            items = [raw]
+        # Drop None explicitly so it doesn't survive as the literal string
+        # "None" (str(None) == "None") — only stringify real values.
+        cleaned: list[str] = []
+        for item in items:
+            if item is None:
+                continue
+            text = str(item).strip()
+            if text:
+                cleaned.append(text)
+        return cleaned
+
     async def _generate(self, task: dict):
-        server_ids: str | None = self.conf.get("server_ids", None)
-        # "channel1,channel2"
-        channel_names: str | None = self.conf.get("channel_names", None)
+        server_ids_raw = self.conf.get("server_ids", None)
+        # Web form stores channels under "channels"; older configs / SDK use
+        # "channel_names" — accept either so existing sources keep working.
+        channels_raw = self.conf.get("channels", None)
+        if channels_raw in (None, "", []):
+            channels_raw = self.conf.get("channel_names", None)
+
+        server_id_strs = self._coerce_str_list(server_ids_raw)
+        # DiscordConnector.__init__ takes server_ids as list[str] and converts
+        # to list[int] internally (common/data_source/discord_connector.py:247).
+        # Validate up-front so a malformed entry warns + drops here rather than
+        # crashing the connector's int() cast — but keep the strings.
+        server_ids: list[str] = []
+        for sid in server_id_strs:
+            try:
+                int(sid)
+            except ValueError:
+                logging.warning("Discord connector: ignoring non-integer server_id %r", sid)
+                continue
+            server_ids.append(sid)
+        channel_names = self._coerce_str_list(channels_raw)
 
         self.connector = DiscordConnector(
-            server_ids=server_ids.split(",") if server_ids else [],
-            channel_names=channel_names.split(",") if channel_names else [],
+            server_ids=server_ids,
+            channel_names=channel_names,
             start_date=datetime(1970, 1, 1, tzinfo=timezone.utc).strftime("%Y-%m-%d"),
             batch_size=self.conf.get("batch_size", 1024),
         )