mssql_merge_exists physical property not respected in python models

[notatyrannosaur]

i’m using sqlmesh with a python model that reads data from pickled dataframes and inserts into a slowly changing dimension (scd type 2) table on mssql server.

i’ve configured the model with:

@model(
    ...
    physical_properties={"mssql_merge_exists": True}
)

however, when the query is rendered in the logs, there is no usage of the MERGE keyword. instead, the generated sql falls back to a large series of WITH ... ctes, unions, and inserts.

this behavior is causing two issues:

1. the model does not run successfully — it crashes after ~20 minutes of execution.

2. sql server resource usage spikes heavily (likely because of the expanded cte/unions strategy instead of a merge).

[themisvaltinos]

fix for this was merged and released: #5544