Fix flaky config test by running tests serially

Cargo.toml

@@ -33,3 +33,6 @@ base64 = "0.22.1"
 # Disable the needless_return lint globally
 # Note: This key does not exist in clippy.toml directly, hence inline attributes are preferred.
 
+[dev-dependencies]
+serial_test = "3.2.0"
+

tests/config_tests.rs

@@ -1,5 +1,6 @@
 use client_rust_fix::config::Settings;
 use std::env;
+use serial_test::serial;
 
 fn setup() {
     // Clear all relevant environment variables before each test
@@ -15,13 +16,15 @@ fn setup() {
 }
 
 #[test]
+#[serial]
 fn test_settings_from_env_defaults() {
     setup();
     let settings = Settings::from_env().expect("load settings");
     assert_eq!(settings.symbol, "SOL-USD");
 }
 
 #[test]
+#[serial]
 fn test_settings_custom_values() {
     setup();