fix(mcp): reject malformed Args on Add MCP Server instead of silently defaulting to [] (#6215)

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

* test(mcp): pass every Form param add_server reads past args validation

CI's pytest run showed test_add_server_still_accepts_valid_json_args and
test_add_server_still_defaults_empty_args_to_empty_list failing with
TypeError: the JSON object must be str, bytes or bytearray, not Form.

Calling the endpoint function directly bypasses FastAPI's dependency
resolution, so an unpassed Form(...) parameter (url, oauth_file,
oauth_config) arrives as the Form marker object itself rather than its
declared default, and add_server's later `if oauth_file:` check reads
that marker as truthy. The malformed-args test never hit this because it
raises before reaching that code. Not a production bug: a real HTTP
request resolves these through FastAPI before add_server ever runs.

* fix(mcp): reject non-list args and surface the new 400 in the Admin panel

o3LL's review on #6215 found two gaps in the args validation this PR adds:
the Admin panel posts to the same /api/mcp/servers endpoint but never
validates Args client-side, so the new 400 falls into the generic failure
branch and shows "Added but connection failed: unknown". Mirror the same
JSON.parse guard settings.js already has.

Also add an isinstance(list) check next to the existing JSON parse, since
valid-but-wrong-shaped JSON (args=5) reaches StdioServerParameters(args=5)
and 500s in the error formatter. Pre-existing on dev, same validation site
this PR already touches.

* fix(admin): surface the server's 400 detail instead of a generic connection-failed message

The Admin add-server handler read needs_oauth/connected/error but never
res.ok, so a request rejected by the isinstance(list) check added for
#6211 (args=5, a valid-JSON-but-non-list value the client-side JSON.parse
guard cannot catch) fell into the same-shape else branch as a successful
add whose connection attempt failed, and the form fields were cleared as
if the server had accepted it.

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 {}