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.

static/js/admin.js

@@ -2366,6 +2366,7 @@ function initMcpForm() {
     if (transport === 'stdio' && !command) { msg.textContent = 'Command is required for stdio'; msg.className = 'admin-error'; return; }
     if (transport === 'sse' && !url) { msg.textContent = 'URL is required for SSE'; msg.className = 'admin-error'; return; }
     try { JSON.parse(env); } catch { msg.textContent = 'Env must be valid JSON'; msg.className = 'admin-error'; return; }
+    try { JSON.parse(args); } catch { msg.textContent = 'Args must be valid JSON, e.g. ["-y", "pkg"]'; msg.className = 'admin-error'; return; }
     const fd = new FormData();
     fd.append('name', name); fd.append('transport', transport); fd.append('command', command); fd.append('args', args); fd.append('env', env); fd.append('url', url);
     // If preset has oauthFile config, send credentials for file generation
@@ -2386,6 +2387,10 @@ function initMcpForm() {
     try {
       const res = await fetch('/api/mcp/servers', { method: 'POST', body: fd, credentials: 'same-origin' });
       const data = await res.json();
+      if (!res.ok) {
+        msg.textContent = data.detail || `Failed (${res.status})`; msg.className = 'admin-error';
+        return;
+      }
       if (data.needs_oauth) {
         msg.innerHTML = `Added ${esc(name)} — <a href="/api/mcp/oauth/authorize/${data.id}" target="_blank" style="color:var(--red);font-weight:600;">Authorize with Google</a> to connect`;
         msg.className = 'admin-success';

static/js/settings.js

@@ -5036,7 +5036,11 @@ async function initUnifiedIntegrations() {
         fd.append('transport', transport);
         if (transport === 'stdio') {
           fd.append('command', el('uf-mcp-cmd').value);
-          let args = '[]'; try { args = JSON.stringify(JSON.parse(el('uf-mcp-args').value || '[]')); } catch (_) {}
+          // Unlike env below, an unparseable args value is not silently
+          // defaulted: it would spawn the subprocess with an empty argv.
+          let args;
+          try { args = JSON.stringify(JSON.parse(el('uf-mcp-args').value || '[]')); }
+          catch (_) { el('uf-mcp-msg').textContent = 'Args must be valid JSON, e.g. ["-y", "pkg"]'; return; }
           let env  = '{}'; try { env  = JSON.stringify(JSON.parse(el('uf-mcp-env').value  || '{}')); } catch (_) {}
           fd.append('args', args);
           fd.append('env', env);