docs: use npx.cmd on Windows in client example

[formulahendry]

When running src/examples/client.ts on Windows, meet with below error:

PS C:\code\typescript-sdk> npx tsx src/examples/client.ts
node:events:497
      throw er; // Unhandled 'error' event
      ^

Error: spawn npx ENOENT
    at ChildProcess._handle.onexit (node:internal/child_process:285:19)
    at onErrorNT (node:internal/child_process:483:16)
    at process.processTicksAndRejections (node:internal/process/task_queues:90:21)
Emitted 'error' event on ChildProcess instance at:
    at ChildProcess._handle.onexit (node:internal/child_process:291:12)
    at onErrorNT (node:internal/child_process:483:16)
    at process.processTicksAndRejections (node:internal/process/task_queues:90:21) {       
  errno: -4058,
  code: 'ENOENT',
  syscall: 'spawn npx',
  path: 'npx',
  spawnargs: [ 'tsx', 'C:\\code\\typescript-sdk\\src\\examples\\agent.ts' ]
}

Node.js v22.19.0

On Windows, npx is installed as a batch script wrapper (npx.cmd), not a native executable. Node.js spawn() without a shell calls the OS directly, which only looks for exact filenames and won't try appending .cmd — causing an ENOENT error. Using npx.cmd explicitly resolves this without the side effects of shell: true.

[benbrandt]

thanks!