Support the Quarto knitr engine when deploying content

Goal

Allow content using the Quarto knitr engine to be deployed and to have a
manifest written. Today validate_quarto_engines (rsconnect/actions.py) accepts
only markdown and jupyter and rejects everything else, including knitr.

Why

knitr is the default Quarto engine for documents that mix R and Python chunks.
Now that rsconnect-python can carry R dependencies from renv.lock, the original
reason for excluding knitr no longer applies, so the rejection blocks a case
the tool can otherwise handle. Raised in review:
https://github.com/posit-dev/rsconnect-python/pull/785#discussion_r3431999679

References

• rsconnect/actions.py (validate_quarto_engines) — the engine allowlist.
• rsconnect/main.py — the four Quarto entry points already branch on
  "jupyter" in engines and handle the no-Python-environment case (markdown).
• renv.lock R-dependency support: PR: support for parsing renv.lock #785

Commencez dans rsconnect/actions.py au niveau de validate_quarto_engines pour retracer la validation actuelle des moteurs, puis examinez les quatre points d’entrée Quarto dans rsconnect/main.py et leurs branches jupyter et markdown existantes. Vérifiez que le déploiement de contenu Quarto utilisant knitr fonctionne, notamment l’écriture du manifeste et le cas sans environnement Python, en vous servant des points d’entrée existants et des vérifications associées comme guide.