fix(mcp): reject malformed Args on Add MCP Server instead of defaulting to []

`add_server` wrapped `json.loads(args)` in a bare `except` that fell back to
`[]`, so an Args value that is not JSON — a bare path, which is what the form's
placeholder invites people to type — registered the server and spawned the
stdio subprocess with an empty argv. Nothing surfaced the loss: the POST
returned 200 and the row persisted with `"args": []`.

The route now returns 400 for an unparseable value, and also for valid JSON of
the wrong shape: `args=5` reached `StdioServerParameters(args=5)` and raised an
unhandled TypeError in the error formatter's `" ".join(...)`.

Both form clients mirror the guard instead of leaving the user to read a 400
they cannot see. `settings.js` stops silently defaulting a bad Args value, and
`admin.js` gains the same client-side parse check plus a `res.ok` branch so a
server-side rejection is not reported as a connection failure.

Ported from public `dev` (`9d5c0319`, #6215 upstream, fixing #6211), with its
test. The `admin.js` hunks are inert on `lab` — `initMcpForm` early-returns
because that form's markup is not in this build — and are carried anyway to
keep the two lines from diverging further.

routes/mcp/mcp_routes.py

@@ -181,10 +181,17 @@ def setup_mcp_routes(mcp_manager: McpManager):
         if transport == "http" and not url:
             raise HTTPException(400, "url is required for HTTP transport")
 
-        # Parse JSON fields
-        try:
-            parsed_args = json.loads(args) if args else []
-        except json.JSONDecodeError:
+        # Parse JSON fields. args is not defaulted on a parse failure: an
+        # unparseable value is silently discarded downstream (stdio spawns
+        # with an empty argv), so the caller must be told instead.
+        if args:
+            try:
+                parsed_args = json.loads(args)
+            except json.JSONDecodeError:
+                raise HTTPException(400, "args must be valid JSON, e.g. [\"-y\", \"pkg\"]")
+            if not isinstance(parsed_args, list):
+                raise HTTPException(400, "args must be a JSON array, e.g. [\"-y\", \"pkg\"]")
+        else:
             parsed_args = []
         try:
             parsed_env = json.loads(env) if env else {}