The model-run maintenance fields (modelRunPruneAfterMs from #88632 base work,
modelRunPruneAfterConfigured from the pressure-gating fix) were required on the
resolved maintenance config exposed to plugins via patchSessionEntry's
maintenanceConfig. External plugin TypeScript callers that construct a
pre-#88632 maintenanceConfig would fail to compile.
Make both fields optional on ResolvedSessionMaintenanceConfig (and the runtime
type), so old-shape plugin configs keep compiling. All internal readers already
treat an absent value as unset: shouldRunModelRunPrune returns false when
modelRunPruneAfterMs == null and modelRunPruneAfterConfigured is falsy, so a
plugin-supplied config without the fields runs no model-run pruning — the
pre-#88632 behavior. The resolver still always populates both fields, so normal
runtime behavior is unchanged. Add an old-shape maintenanceConfig SDK
regression test.
