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

Start in rsconnect/actions.py at validate_quarto_engines to trace the current engine validation, then inspect the four Quarto entry points in rsconnect/main.py and their existing jupyter and markdown branches. Confirm deployment works for Quarto content using knitr, including manifest writing and the no-Python-environment case, using the existing entry points and related checks as guidance.