Studio: move MCP to a composer button with presets (#5926)

Move MCP from the chat settings sidebar to a composer pill next to Artifacts. The dropdown lists keyless presets (Context7, Exa, Hugging Face), all off by default, created on first enable and deduped by URL. Add custom MCP opens the existing manage dialog. Enabled rows show a green underlay and a tick that becomes an X on hover to remove; enabling Exa turns off the built-in Web Search and shows a hover tooltip. Reconciles the per-chat MCP flag on load so already-enabled servers activate the pill. No backend changes.

studio/frontend/src/components/assistant-ui/thread.tsx

@@ -44,6 +44,7 @@ import {
 } from "@/components/ui/dropdown-menu";
 import { sentAudioNames } from "@/features/chat/api/chat-adapter";
 import { parseExternalModelId } from "@/features/chat/external-providers";
+import { McpComposerButton } from "@/features/chat/mcp-composer-button";
 import { getExternalReasoningCapabilities } from "@/features/chat/provider-capabilities";
 import { useChatRuntimeStore } from "@/features/chat/stores/chat-runtime-store";
 import { useExternalProvidersStore } from "@/features/chat/stores/external-providers-store";
@@ -1215,6 +1216,7 @@ const ComposerAction: FC<{
         <CodeToolsToggle />
         <ImagesToggle />
         <ArtifactsToggle />
+        <McpComposerButton />
       </div>
       <div className="flex items-center gap-1">
         <ComposerPrimitive.If dictation={false}>