[mcanouil]

Several extensions read the QUARTO_EXECUTE_INFO file themselves to get the project's website and book metadata, each with its own error handling.
This adds get_project_metadata to metadata.lua, so they can share one tested reader.

It returns the format.metadata table.
When the variable is set but the file can't be used, it returns nil and one of 'file is unreadable', 'file is empty', 'invalid JSON' or 'no format.metadata table'.
When the variable is unset or empty, it returns nil alone, so single documents stay silent.
The module doesn't log or cache, and each extension decides whether to warn.

A decoder that returns nil counts as invalid JSON, so quarto-cli and q2 report the same reason.

get_project_repo_url now builds on it and takes an optional format_meta table, so an extension that already read the file doesn't read it again.
A side effect: JSON that isn't an object now gives nil instead of an error that stopped the filter.

The tests stub os.getenv and quarto.json and cover each failure path, website only, book only and both.