Function load_meta_from_str 

pub fn load_meta_from_str(content: &str) -> Result<OAuthMeta, Error>

Parse the OAuth sections from raw TOML.

The wrapping shape is:

[providers.<name>.oauth]
client_id = "..."
# ...

Providers without an [providers.<name>.oauth] table are dropped from the result — empty/missing block = key-only provider.