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.

static/js/admin.js

@@ -3142,6 +3142,7 @@ 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
@@ -3162,6 +3163,10 @@ 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

@@ -5102,7 +5102,11 @@ 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);