[ACTP] address review feedback on launch config and idle timeout docs

Treat an empty PAR boolean environment variable as unset so it falls back to
YAML or the default instead of aborting startup, matching how the fleet dir and
log level are already resolved.

Correct the idle_timeout_seconds description: the executor now self-terminates
after exactly this many seconds, not three times as many.

Author: embeaken

pkg/config/schema/yaml/private_action_runner.yaml

@@ -122,9 +122,9 @@ properties:
     type: integer
     default: 60
     description: |-
-      Sets the split-mode control plane's executor idle timeout in seconds. The
-      executor uses three times this value as a self-termination watchdog;
-      values of zero or less disable the watchdog.
+      Sets the split-mode executor's idle timeout in seconds. The executor
+      self-terminates after this much time without activity; values of zero or
+      less disable the watchdog.
   log_file:
     node_type: setting
     type: string

pkg/privateactionrunner/par-control/src/config.rs

@@ -140,7 +140,7 @@ fn resolve_bool(
     if fleet_value.is_some() {
         return Ok(fleet_value);
     }
-    let Some(raw) = env(name) else {
+    let Some(raw) = env(name).filter(|value| !value.is_empty()) else {
         return Ok(yaml_value);
     };
     let value = match raw.trim() {
@@ -195,6 +195,23 @@ mod tests {
         assert_eq!(launch.log_level, log::LevelFilter::Trace);
     }
 
+    #[test]
+    fn empty_environment_overrides_fall_back_to_yaml() {
+        let env = |name: &str| match name {
+            "DD_PRIVATE_ACTION_RUNNER_ENABLED"
+            | "DD_PRIVATE_ACTION_RUNNER_SPLIT_ENABLED"
+            | "DD_PRIVATE_ACTION_RUNNER_SELF_ENROLL" => Some(String::new()),
+            _ => None,
+        };
+        let launch = Launch::from_yaml_str_with_env(
+            "private_action_runner:\n  enabled: true\n  split_enabled: true\n  self_enroll: false\n",
+            env,
+        )
+        .unwrap();
+        assert!(launch.gate.split_mode);
+        assert!(!launch.gate.self_enroll);
+    }
+
     #[test]
     fn fleet_policy_overrides_local_config_and_environment() {
         let dir = tempfile::tempdir().unwrap();