Make sample message rule more relevant based on entity

**Case:**
Currently in sample message rule independent of for which entity type(Individual/Encounter/Enrolment) it is for, it has the below line: ```const individual = params.entity;```

Instead if it is for encounter, the code should have been ```const encounter = params.entity;``` like we have already done in visit schedule rule and decision rule.

**Need to fix:**
For self-implementing organisations and implementation team, this will help to understand better how to access the entity.

Find the sample message rule and compare its entity handling with the existing visit schedule rule and decision rule. Update the example so the variable matches the entity type being handled, and verify that Individual, Encounter, and Enrolment examples access their entities consistently.

Written by the indexing model from the issue text.