feat(mcp): simplify external MCP server config to local/remote modes

Replace the three-way transport choice (stdio / sse / httpstream) for
connecting LangBot to external MCP servers with two modes: local (stdio)
and remote. Remote servers only require a URL; the runtime auto-detects
the transport (tries Streamable HTTP, falls back to SSE).

- provider/tools/loaders/mcp.py: add _init_remote_server() with
  Streamable-HTTP-then-SSE probing; dispatch 'remote' lifecycle, keep
  legacy sse/http branches for back-compat
- plugin/connector.py: normalize legacy http/sse marketplace modes to
  'remote' on Space install, preserving connection params
- entity/persistence/mcp.py: document mode as stdio, remote (legacy: sse, http)
- alembic 0006: idempotent data migration mapping existing sse/http rows
  to remote (downgrade maps back to http)
- api/http/service/mcp.py: stash runtime_info (status + tool list) into
  test task metadata before tearing down the temp session
- web: collapse mode dropdown to local/remote, remote renders URL+timeout
  only, edit auto-maps legacy sse/http to remote; show tools after test in
  create mode from task metadata; remove dead plugins/mcp-server/ tree
- i18n: local/remote labels + mode/url hints across 8 locales

web/src/app/infra/entities/api/index.ts

@@ -531,6 +531,15 @@ export interface MCPServerExtraArgsHttp {
   timeout: number;
 }
 
+// "remote" mode: the user only supplies a URL; the backend auto-detects the
+// transport (Streamable HTTP first, falling back to legacy SSE). headers /
+// timeout are optional advanced settings.
+export interface MCPServerExtraArgsRemote {
+  url: string;
+  headers?: Record<string, string>;
+  timeout?: number;
+}
+
 export enum MCPSessionStatus {
   CONNECTING = 'connecting',
   CONNECTED = 'connected',
@@ -577,6 +586,17 @@ export type MCPServer =
       created_at?: string;
       updated_at?: string;
     }
+  | {
+      uuid?: string;
+      name: string;
+      mode: 'remote';
+      enable: boolean;
+      extra_args: MCPServerExtraArgsRemote;
+      runtime_info?: MCPServerRuntimeInfo;
+      readme?: string;
+      created_at?: string;
+      updated_at?: string;
+    }
   | {
       uuid?: string;
       name: string;