Function classify_runtime_env_var 

pub fn classify_runtime_env_var(
    value: Option<&str>,
    from: &EnvId,
) -> Option<MigrationFinding>

Pure classification helper for RuntimeConfigScanner. Returns Some(finding) iff value equals the legacy env id. Extracted from the scanner so tests can exercise the logic without mutating the process environment — the crate is #![forbid(unsafe_code)] and Rust 2024 requires unsafe to set/remove env vars.