Update eng/tools/typespec-python-generated-tests/README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: iscai-msft

eng/tools/typespec-python-generated-tests/README.md

@@ -20,7 +20,9 @@ To regenerate manually, trigger the workflow via GitHub Actions or run locally:
 
 ```bash
 # From the microsoft/typespec repo (packages/http-client-python):
+npm run install
 npm run build
+npm run prepare
 npm run regenerate
 
 # Then copy tests/generated/{azure,unbranded} to this folder