test: add unit tests for auth middleware cookie extraction

Extract cookie parsing into a standalone `extract_session_token` function
and add 5 unit tests covering the valid, missing, multi-cookie, whitespace,
and empty-header cases.

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

backend/src/middleware/auth.rs

@@ -28,6 +28,20 @@ pub struct AuthUser {
     pub role: UserRole,
 }
 
+/// Extract session token from a Cookie header value.
+///
+/// Splits the header on `;`, trims whitespace, and returns the value of the
+/// first cookie whose name matches `cookie_name`.  Returns `None` when no
+/// matching cookie is present.
+fn extract_session_token(cookie_header: &str, cookie_name: &str) -> Option<String> {
+    cookie_header
+        .split(';')
+        .map(|s| s.trim())
+        .find(|s| s.starts_with(&format!("{}=", cookie_name)))
+        .and_then(|s| s.strip_prefix(&format!("{}=", cookie_name)))
+        .map(|s| s.to_string())
+}
+
 impl FromRequestParts<AppState> for AuthUser {
     type Rejection = AppError;
 
@@ -36,7 +50,6 @@ impl FromRequestParts<AppState> for AuthUser {
         state: &AppState,
     ) -> impl std::future::Future<Output = Result<Self, Self::Rejection>> + Send {
         let pool = state.pool.clone();
-        let cookie_prefix = format!("{}=", SESSION_COOKIE_NAME);
 
         // Extract the session cookie value from the Cookie header
         let session_token = parts
@@ -44,11 +57,7 @@ impl FromRequestParts<AppState> for AuthUser {
             .get_all(header::COOKIE)
             .iter()
             .filter_map(|v| v.to_str().ok())
-            .flat_map(|s| s.split(';'))
-            .map(|s| s.trim())
-            .find(|s| s.starts_with(&cookie_prefix))
-            .and_then(|s| s.strip_prefix(&cookie_prefix))
-            .map(|s| s.to_string());
+            .find_map(|header_value| extract_session_token(header_value, SESSION_COOKIE_NAME));
 
         async move {
             let session_token = session_token
@@ -102,3 +111,50 @@ impl FromRequestParts<AppState> for AdminUser {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::extract_session_token;
+    use crate::services::auth::SESSION_COOKIE_NAME;
+
+    #[test]
+    fn test_valid_session_cookie_extracted() {
+        let header = format!("{}=abc123def", SESSION_COOKIE_NAME);
+        assert_eq!(
+            extract_session_token(&header, SESSION_COOKIE_NAME),
+            Some("abc123def".to_string())
+        );
+    }
+
+    #[test]
+    fn test_missing_session_cookie_returns_none() {
+        let header = "other=value; another=thing";
+        assert_eq!(extract_session_token(header, SESSION_COOKIE_NAME), None);
+    }
+
+    #[test]
+    fn test_multiple_cookies_correct_one_picked() {
+        let header = format!("theme=dark; {}=mytoken123; lang=fr", SESSION_COOKIE_NAME);
+        assert_eq!(
+            extract_session_token(&header, SESSION_COOKIE_NAME),
+            Some("mytoken123".to_string())
+        );
+    }
+
+    #[test]
+    fn test_cookie_with_whitespace_trimmed() {
+        // The whole segment is trimmed before matching, so leading/trailing
+        // whitespace around name=value pairs is stripped.  The value itself
+        // also has its trailing spaces removed as part of that segment trim.
+        let header = format!("  {}=abc123  ; other=val  ", SESSION_COOKIE_NAME);
+        assert_eq!(
+            extract_session_token(&header, SESSION_COOKIE_NAME),
+            Some("abc123".to_string())
+        );
+    }
+
+    #[test]
+    fn test_empty_cookie_header_returns_none() {
+        assert_eq!(extract_session_token("", SESSION_COOKIE_NAME), None);
+    }
+}