{bulk,modal}_aerosol_state_mod.F90: hardcoded rair 287.04

### Issue Type

Code Clean-up

### Issue Description

Need to replace hardcoded instances (grep `287.04`) with the actual gas constant for the host model.

### Will this change answers?

Yes

### Will you be implementing this yourself?

Any Software Engineer can do this

Start with bulk_aerosol_state_mod.F90 and modal_aerosol_state_mod.F90, then grep the repository for 287.04. Trace how the host model exposes its gas constant and replace the hardcoded instances with that value. Done means no affected hardcoded 287.04 values remain and the model still answers consistently.

Written by the indexing model from the issue text.