feat: add specialized observed for AbstractDDEIntegrator

Author: AayushSabharwal

src/integrator_interface.jl

@@ -910,3 +910,11 @@ Checks if the integrator is adaptive
 function isadaptive(integrator::DEIntegrator)
     isdefined(integrator.opts, :adaptive) ? integrator.opts.adaptive : false
 end
+
+function SymbolicIndexingInterface.observed(integ::AbstractDDEIntegrator, sym)
+    return let histfn = DDESolutionHistoryWrapper(get_sol(integ)),
+        rawfn = SymbolicIndexingInterface.observed(symbolic_container(integ), sym)
+
+        fn(u, p, t) = rawfn(u, histfn, p, t)
+    end
+end