Address PR #1204 review: add ajv dep and StdioServerConfig comment

Add ajv as an explicit dependency in clients/web/package.json (was only
available transitively via @modelcontextprotocol/sdk). Add inline comment
on StdioServerConfig.type explaining the implicit-default contract and
its impact on switch narrowing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/mcp/types.ts

@@ -16,6 +16,9 @@ import type { JsonValue } from "../json/jsonUtils.js";
 
 // Stdio transport config
 export interface StdioServerConfig {
+  // Optional: stdio is the implicit default when `type` is absent. A
+  // narrowing `switch (config.type)` must therefore cover the `undefined`
+  // branch as `StdioServerConfig`.
   type?: "stdio";
   command: string;
   args?: string[];