Incorrectly deserializes upper case unquoted `INF` string value from a YAML file

Description

Here is the minimal test set to reproduce the issue

Source Files

assets/config.yaml

val: INF

src/lib.rs

#[cfg(test)]
mod tests {
    use config::Config;
    use serde::Deserialize;

    #[test]
    fn test_yaml_inf() {
        #[derive(Deserialize)]
        struct Settings {
            pub val: String,
        }

        let cfg = Config::builder()
            .add_source(config::File::with_name("assets/config.yaml"))
            .build()
            .unwrap();
        let settings = cfg.try_deserialize::<Settings>().unwrap();

        assert_eq!(settings.val, "INF");
    }
}

Cargo.toml

[package]
name = "hl-issue-288"
version = "0.1.0"
edition = "2021"

[dependencies]
config = { version = "0", features = ["yaml"] }
serde = { version = "1", features = ["derive"] }

Expected results

• The INF value is deserialized to a String as "INF"
• The test passes

Actual results

• The INF value is deserialized to a String as "inf"
• The test fails

Notes

• The problem is that INF is deserialized to some internal representation as a float +Inf value before it is deserialized to a String.

Start with the minimal reproducer in src/lib.rs and assets/config.yaml, using the Cargo.toml dependencies shown in the issue. Run the test to confirm that unquoted INF becomes "inf" instead of "INF", then trace the YAML source through deserialization. Done means the test passes while preserving the exact uppercase string.