petab.get_model_for_condition may remove rateRules where it shouldn't

If a rate-rule target occurs in the condition table, this should only affect the initial value, not the rule itself.

Reported by @dilpath

Start by locating petab.get_model_for_condition and trace how rate-rule targets from the condition table are handled. Verify that a matching target changes only its initial value while preserving the rate rule, then add or update coverage for that behavior if the surrounding tests identify an appropriate location.