fix(config): Fix env var priority over file config (#467)

The `load_envs` function was merging in the wrong order, passing the
file-based config as the argument to `merge`. In schematic, the argument
to `merge` takes precedence over the receiver, so this caused file
configuration to silently override environment variables — the opposite
of the intended behavior.

The fix swaps the receiver and argument: environment variables now merge
into the file-based config, correctly taking precedence over any
file-level settings.

Signed-off-by: Jean Mertz <git@jeanmertz.com>

Author: JeanMertz

crates/jp_config/src/util.rs

@@ -53,12 +53,12 @@ pub fn load_partials_with_inheritance(
 ///
 /// Returns an error if merging the partials fails, which returns a
 /// [`schematic::MergeError`].
-pub fn load_envs(base: PartialAppConfig) -> Result<PartialAppConfig, BoxedError> {
+pub fn load_envs(mut base: PartialAppConfig) -> Result<PartialAppConfig, BoxedError> {
     trace!("Loading environment variable configuration.");
-    let mut partial = PartialAppConfig::from_envs()?;
-    partial.merge(&(), base)?;
+    let envs = PartialAppConfig::from_envs()?;
+    base.merge(&(), envs)?;
 
-    Ok(partial)
+    Ok(base)
 }
 
 /// Tries to find a configuration file in a load path.

crates/jp_config/src/util_tests.rs

@@ -95,6 +95,22 @@ fn test_load_envs() {
     );
 }
 
+#[test]
+#[serial(env_vars)]
+fn test_load_envs_overrides_file_config() {
+    let _env = EnvVarGuard::set("JP_CFG_PROVIDERS_LLM_OPENROUTER_API_KEY_ENV", "FROM_ENV");
+
+    let mut file_config = PartialAppConfig::empty();
+    file_config.providers.llm.openrouter.api_key_env = Some("FROM_FILE".to_owned());
+
+    let merged = load_envs(file_config).unwrap();
+    assert_eq!(
+        merged.providers.llm.openrouter.api_key_env,
+        Some("FROM_ENV".to_owned()),
+        "environment variables should override file config"
+    );
+}
+
 #[test]
 fn test_build() {
     let error = build(PartialAppConfig::default_values(&()).unwrap().unwrap()).unwrap_err();