test: add MCP client e2e tests with real MCP servers

Prove the full MCP protocol flow works end-to-end against real servers:
- Self-written fixture server: tool discovery, execution, error handling,
  image placeholder, toolPrefix, and clean disposal
- @modelcontextprotocol/server-everything: echo, get-sum, get-tiny-image
- @modelcontextprotocol/server-filesystem: write_file + read_file round-trip,
  list_directory with world-verification

All 15 tests keyless and deterministic (no API key needed).

packages/mcp/mcp-client/package.json

@@ -33,6 +33,9 @@
   "devDependencies": {
     "@deepseek-ai/dsh-tools": "workspace:^",
     "@deepseek-ai/dsh-llm": "workspace:^",
-    "cordis": "^4.0.0-rc.6"
+    "@modelcontextprotocol/server-everything": "^2026.7.4",
+    "@modelcontextprotocol/server-filesystem": "^2026.7.4",
+    "cordis": "^4.0.0-rc.6",
+    "zod": "^4.4.3"
   }
 }