fix: use OsString for EnvGuard and add remove() constructor

Addresses PR review feedback:
- Store previous env var as OsString (via env::var_os) to correctly
  handle non-UTF-8 values instead of silently dropping them.
- Add EnvGuard::remove() for tests that need to ensure a variable
  is unset, so test_envsubst_missing also gets proper cleanup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mikkeldamsgaard

src/render.rs

@@ -77,20 +77,31 @@ pub fn template_render(input: &str) -> Result<String, String> {
 mod tests {
     use super::*;
 
+    use std::ffi::OsString;
+
     struct EnvGuard {
         name: String,
-        previous: Option<String>,
+        previous: Option<OsString>,
     }
 
     impl EnvGuard {
         fn set(name: &str, value: &str) -> Self {
-            let previous = env::var(name).ok();
+            let previous = env::var_os(name);
             env::set_var(name, value);
             Self {
                 name: name.to_string(),
                 previous,
             }
         }
+
+        fn remove(name: &str) -> Self {
+            let previous = env::var_os(name);
+            env::remove_var(name);
+            Self {
+                name: name.to_string(),
+                previous,
+            }
+        }
     }
 
     impl Drop for EnvGuard {
@@ -110,7 +121,7 @@ mod tests {
     }
     #[test]
     fn test_envsubst_missing() {
-        env::remove_var("MISSING_VAR_XYZ");
+        let _g = EnvGuard::remove("MISSING_VAR_XYZ");
         assert_eq!(envsubst("${MISSING_VAR_XYZ}"), "${MISSING_VAR_XYZ}");
     }
     #[test]