Ensure workload_size is marked as non-computed for databricks_model_serving (#5111)

## Changes
`workload_size` is currently marked as `optional` and `computed`. As a
result, if a user sets a model serving endpoint using `workload_size`
initially, then removes it in favor of
`min_provisioned_concurrency`/`max_provisioned_concurrency`,
`workload_size` remains in the state. As a result, it is also included
in the request sent to the REST API, which is invalid.

By marking this field as non-computed, it should be removed from the
plan when a user removes it from their config. This means it won't be
sent to the REST API.

## Tests
- [ ] Manual test: created a model serving endpoint with `workload_size`
set, then removed it and set min/max provisioned concurrency fields, and
the apply succeeded.

---------

Co-authored-by: Alex Ott <[email protected]>

Author: mgyucht

NEXT_CHANGELOG.md

@@ -10,6 +10,7 @@
 
 ### Bug Fixes
 
+* Mark `workload_size` as not computed in `databricks_model_serving` ([#5111](https://github.com/databricks/terraform-provider-databricks/pull/5111))
 * Fix Inconsistent Plan Errors in Permissions Resource ([#5091](https://github.com/databricks/terraform-provider-databricks/pull/5091))
 
 ### Documentation

serving/resource_model_serving.go

@@ -154,13 +154,14 @@ func ResourceModelServing() common.Resource {
 
 			common.CustomizeSchemaPath(m, "config", "served_models", "name").SetComputed()
 			common.CustomizeSchemaPath(m, "config", "served_models", "workload_type").SetComputed()
+			common.CustomizeSchemaPath(m, "config", "served_models", "workload_size").SetSuppressDiff()
 			common.CustomizeSchemaPath(m, "config", "served_models", "scale_to_zero_enabled").SetOptional().SetDefault(true)
 			common.CustomizeSchemaPath(m, "config", "served_models").SetDeprecated("Please use 'config.served_entities' instead of 'config.served_models'.")
 			common.CustomizeSchemaPath(m, "rate_limits").SetDeprecated("Please use AI Gateway to manage rate limits.")
 
 			common.CustomizeSchemaPath(m, "config", "served_entities", "name").SetComputed()
-			common.CustomizeSchemaPath(m, "config", "served_entities", "workload_size").SetComputed()
 			common.CustomizeSchemaPath(m, "config", "served_entities", "workload_type").SetComputed()
+			common.CustomizeSchemaPath(m, "config", "served_entities", "workload_size").SetSuppressDiff()
 
 			// Apply custom suppress diff to traffic config routes for served_model_name and served_entity_name
 			common.CustomizeSchemaPath(m, "config", "traffic_config", "routes", "served_model_name").SetCustomSuppressDiff(suppressRouteModelEntityNameDiff)