Fix Chinese translation workflow

[B67687]

What changed

This PR fixes the failing Chinese translation workflow and hardens the translation script so the workflow can run reliably.

Root cause

The current workflow calls python translate-notebooks.py, but the repository file is named translate_notebooks.py. The latest failure on April 9, 2026 stopped immediately with:

python: can't open file '/home/runner/work/MathLearningNotes/MathLearningNotes/translate-notebooks.py': [Errno 2] No such file or directory

Additional fixes included

• Update the workflow to use current GitHub Actions versions.
• Run on pull_request for validation, but only push translated notebook updates on non-PR runs.
• Skip recursive bot-triggered reruns.
• Commit translated output only when translated-notebooks actually changed.
• Replace the placeholder Qwen endpoint logic with a real OpenAI-compatible DashScope request path, with env overrides for endpoint and model.
• Stop the job loudly if the translation API fails, instead of silently writing untranslated copies.
• Skip translating notebooks inside translated-notebooks to avoid recursive retranslation.

Validation

• I inspected the latest failed Actions run 24190258737 and confirmed the filename mismatch from the job log.
• I syntax-checked translate_notebooks.py locally.
• I did not run a live translation request locally because that would depend on repository secrets and external API access.