t355: give the withDefaults() coverage test a value for idleSleepGuard

FleetConfigWithDefaultsPreservesEveryComponentTest was added on main after
the sleep-guard commit was cherry-picked, specifically to catch this exact
rebase hazard (a withDefaults() call silently rebinding to a stale-arity
back-compat constructor). Its baseValues() map didn't know about the new
idleSleepGuard component yet, so the coverage test itself failed the
name-drift check. Add a real, non-null value for it, consistent with how
the sibling ConfigRefTopLevelReportingCoverageTest already covers it.

fleetd/src/test/java/dev/ltms/fleet/config/FleetConfigWithDefaultsPreservesEveryComponentTest.java

@@ -46,7 +46,7 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
  * comments document that it only ever REPLACES a component when the incoming value is {@code null}
  * (or blank, for {@code placement}) — {@code broker}/{@code primary}/{@code leadHeartbeat}/
  * {@code configReload}/{@code coordinator}/{@code worktreeGroup}/{@code memberLoginShell}/
- * {@code memberSkills} are left as-is unconditionally, and {@code bind}/{@code guard}/{@code lifecycle}/{@code auth}/
+ * {@code memberSkills}/{@code idleSleepGuard} are left as-is unconditionally, and {@code bind}/{@code guard}/{@code lifecycle}/{@code auth}/
  * {@code fleet}/{@code quarantineCooldownSeconds}/{@code memberCredentials}/{@code placement} are
  * replaced only on null/blank input. A value that is never null or blank going in must therefore
  * never change coming out, for every current component. No exclusion is needed today.
@@ -96,6 +96,7 @@ class FleetConfigWithDefaultsPreservesEveryComponentTest {
         v.put("worktreeGroup", "group-guard");
         v.put("memberLoginShell", "/bin/zsh");
         v.put("memberSkills", "/skills/guard");
+        v.put("idleSleepGuard", new FleetConfig.IdleSleepGuard(true));
         assertNamesMatchComponents(v);
         return v;
     }