fix: only examine RHS in equation_dependencies

Symbolics@5fe7a2d6 added a `search_variables!` method for `Equation`
that searches both LHS and RHS. This broke `equation_dependencies`
which relies on finding only RHS dependencies.

The original MTK behavior (from 2020, commit 3160770) explicitly
only examined `eq.rhs`. This fix restores that behavior by passing
`eq.rhs` directly instead of relying on the generic `Equation` method.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: SebastianM-C

lib/ModelingToolkitBase/src/systems/dependency_graphs.jl

@@ -44,7 +44,7 @@ function equation_dependencies(
     depeqs_to_vars = Vector{Vector}(undef, length(eqs))
 
     for (i, eq) in enumerate(eqs)
-        get_variables!(deps, eq, variables)
+        get_variables!(deps, eq.rhs, variables)
         depeqs_to_vars[i] = [value(v) for v in deps]
         empty!(deps)
     end