feat: export generated schemas as public API

Add `@modelcontextprotocol/sdk/generated` export so users can import
auto-generated schemas directly:

```typescript
import { ProgressTokenSchema } from '@modelcontextprotocol/sdk/generated';
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ochafik

package.json

@@ -51,6 +51,10 @@
             "import": "./dist/esm/experimental/tasks/index.js",
             "require": "./dist/cjs/experimental/tasks/index.js"
         },
+        "./generated": {
+            "import": "./dist/esm/generated/index.js",
+            "require": "./dist/cjs/generated/index.js"
+        },
         "./*": {
             "import": "./dist/esm/*",
             "require": "./dist/cjs/*"