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

rsconnect/actions.py の validate_quarto_engines から始めて現在のエンジン検証を追跡し、次に rsconnect/main.py にある4つの Quarto エントリポイントと、既存の jupyter ブランチおよび markdown ブランチを調べます。既存のエントリポイントと関連するチェックを手がかりに、knitr を使用する Quarto コンテンツのデプロイが、マニフェストの書き込みと Python 環境がない場合を含めて機能することを確認します。