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

Comienza en rsconnect/actions.py, en validate_quarto_engines, para seguir la validación actual de motores; después, inspecciona los cuatro puntos de entrada de Quarto en rsconnect/main.py y sus ramas existentes para jupyter y markdown. Confirma que la implementación de contenido de Quarto usando knitr funciona, incluida la escritura del manifiesto y el caso sin entorno de Python, tomando como guía los puntos de entrada existentes y las comprobaciones relacionadas.