Fix ADU duct leakage inflating system return flow when no return plenum

[yujiex]

Pull request overview

• Fixes ADU duct leakage without a return plenum causes excessive supply-airflow reduction through inconsistent return-flow accounting #11857

Description of the purpose of this PR

Simple duct leakage from ZoneHVAC:AirDistributionUnit was always added to AirLoopFlow.LeakFlow, and therefore to SysRetFlow, even when the leaked air never reaches a return node. Only a connected AirLoopHVAC:ReturnPlenum adds the leak flow back into the return path. Without one, MixedAir computed the exhaust flow from the inflated SysRetFlow but the mixed flow from the actual return node flow, so the mixed air flow came up short by the leak flow on every HVAC iteration. The shortfall fed back through the available-flow limits on the supply fan and terminal, and the ADU leakage was then applied to the already reduced flow. In a VAV model with 26% leakage, delivered airflow fell by up to ~86% and annual site energy dropped by ~17% instead of rising.

• CalcZoneMassBalance now counts upstream and downstream duct leakage in LeakFlow only if the ADU's zone has a return node feeding a return plenum (new ZonePlenum::zoneReturnFeedsPlenum). Leakage stays active and is replaced through the outdoor air path.
• Remove the check in InitAirZoneReturnPlenum that zeroed leakage for ADUs without a plenum. It only ran when a model had a plenum, and only tested downstream leakage, so models with no plenum never reached it.
• Add a one-time warning per ADU (upstream or downstream leakage, any model) when the ADU's zone has no return plenum, stating that the leaked air is treated as lost from the air loop.

Results are unchanged for models whose leaking ADUs have a return plenum. Models with duct leakage and no plenum will change, and ADUs without a plenum in models that do have one no longer have downstream leakage disabled.

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