Fix incorrect SetSuppressDiff() for workload_size in databricks_model_serving (#5152)

## Changes

This PR removes the `SetSuppressDiff()` calls for `workload_size` that
were added in #5111 (commit 1617350).

The previous PR (#5111) made a faulty assumption that `workload_size`
would be returned by the API when
`min_provisioned_concurrency`/`max_provisioned_concurrency` were
specified. As a result, it added `SetSuppressDiff()` to prevent
Terraform from detecting changes.

However, the API actually does **not** return `workload_size` when
`min_provisioned_concurrency`/`max_provisioned_concurrency` are set.
This means that no diff suppression is needed, and these fields can be
simply treated as optional (which is the default behavior).

## Tests

- [x] Manual test: Verify that switching between `workload_size` and
`min_provisioned_concurrency`/`max_provisioned_concurrency` works
correctly without unexpected diffs
When adding the concurrency parameters and removing workload size:
```
Terraform will perform the following actions:

  # databricks_model_serving.llama_endpoint will be updated in-place
  ~ resource "databricks_model_serving" "llama_endpoint" {
        id                  = "llama-3-2-1b-instruct-endpoint"
        name                = "llama-3-2-1b-instruct-endpoint"
        # (3 unchanged attributes hidden)

      ~ config {
          ~ served_entities {
              ~ max_provisioned_concurrency = 0 -> 4
                name                        = "llama_v3_2_1b_instruct-2"
              - workload_size               = "Small" -> null
                # (10 unchanged attributes hidden)
            }

            # (1 unchanged block hidden)
        }
    }
```

And vice-versa:
```
  # databricks_model_serving.llama_endpoint will be updated in-place
  ~ resource "databricks_model_serving" "llama_endpoint" {
        id                  = "llama-3-2-1b-instruct-endpoint"
        name                = "llama-3-2-1b-instruct-endpoint"
        # (3 unchanged attributes hidden)

      ~ config {
          ~ served_entities {
              - max_provisioned_concurrency = 4 -> null
                name                        = "llama_v3_2_1b_instruct-2"
              + workload_size               = "Small"
                # (10 unchanged attributes hidden)
            }

            # (1 unchanged block hidden)
        }
```
The applies complete without error, and the plan is empty after apply.

## Related

- Previous PR: #5111
- Previous commit:
1617350

---------

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

Author: mgyucht

NEXT_CHANGELOG.md

@@ -10,6 +10,8 @@
 
 ### Bug Fixes
 
+* Remove unnecessary `SetSuppressDiff()` for `workload_size` in `databricks_model_serving` ([#5152](https://github.com/databricks/terraform-provider-databricks/pull/5152)).
+
 ### Documentation
 
 ### Exporter

serving/resource_model_serving.go

@@ -131,6 +131,47 @@ func updateAiGateway(ctx context.Context, w *databricks.WorkspaceClient, name st
 	return err
 }
 
+// cleanWorkloadSize clears the workload_size field from the config (the API response) if it is not set in the corresponding schema.ResourceData.
+// This is applied to both the ServedModels and ServedEntities fields.
+//
+// If neither workload_size nor min_provisioned_concurrency/max_provisioned_concurrency are provided in API requests, workload_size is set in the
+// API response. This results in a configuration drift for workload_size.
+//
+// The resulting behavior is:
+//
+// - If the workload_size is set in the ResourceData, the provider respects the value specified in the API response.
+// - If the workload_size is not set in the ResourceData, the provider clears the workload_size from the API response.
+func cleanWorkloadSize(s map[string]*schema.Schema, d *schema.ResourceData, apiResponse *serving.EndpointCoreConfigOutput) {
+	var config serving.CreateServingEndpoint
+	common.DataToStructPointer(d, s, &config)
+
+	if config.Config == nil {
+		return
+	}
+	for _, configModel := range config.Config.ServedModels {
+		if configModel.WorkloadSize != "" {
+			continue
+		}
+		for i, apiModel := range apiResponse.ServedModels {
+			if apiModel.Name == configModel.Name {
+				apiResponse.ServedModels[i].WorkloadSize = ""
+				break
+			}
+		}
+	}
+	for _, configEntity := range config.Config.ServedEntities {
+		if configEntity.WorkloadSize != "" {
+			continue
+		}
+		for i, apiEntity := range apiResponse.ServedEntities {
+			if apiEntity.Name == configEntity.Name {
+				apiResponse.ServedEntities[i].WorkloadSize = ""
+				break
+			}
+		}
+	}
+}
+
 func ResourceModelServing() common.Resource {
 	s := common.StructToSchema(
 		serving.CreateServingEndpoint{},
@@ -154,14 +195,12 @@ 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_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)
@@ -234,6 +273,8 @@ func ResourceModelServing() common.Resource {
 					endpoint.Config.ServedEntities = nil
 				}
 			}
+			cleanWorkloadSize(s, d, endpoint.Config)
+
 			err = common.StructToData(*endpoint, s, d)
 			if err != nil {
 				return err