Better error when no manifest.json: don't assume Python content

[nealrichardson]

What happened

Deploying an R project (with renv.lock) without a manifest.json, the deploy step ran generate-requirements.sh and failed with a Python-specific error:

No uv.lock or pyproject.toml file found. Please run 'uv sync' to generate uv.lock or create a pyproject.toml before deploying.
##[error]Process completed with exit code 1.

The action appears to assume Python content whenever manifest.json is absent. For an R project this error is confusing and misleading — it points the user toward uv/pyproject.toml when the actual requirement is an R manifest.

Expected behavior

• Don't assume Python content solely because manifest.json is missing.
• Detect content type (e.g. presence of renv.lock, .R/.Rmd/.qmd files, DESCRIPTION, etc.) before choosing the requirements-generation path.
• Produce an error message tailored to the detected content type. For R content, the message should explain that a manifest.json is required (and how to generate one) rather than mentioning uv.lock/pyproject.toml.