Bugfix conversion of single activities in pipeline json files

[Alex-Swann]

Bugfix conversion of single activities in pipeline json files when handling dynamic variables.

If a pipeline has a variable that needs to be dynamic to accommodate this being utilised in different deployment environments (DEV, UAT, PROD), e.g.

{
	"name": "Run all notebooks",
	"properties": {
		"activities": [
			{
				"name": "ForEach Notebook",
				"type": "ForEach",
				"dependsOn": [],
				"userProperties": [],
				"typeProperties": {
					"items": {
						"value": "<an_array_of_string_values_exposed_via_@item()>",
						"type": "Expression"
					},
					"activities": [
						{
							"name": "Child - Pipeline",
							"type": "ExecutePipeline",
							"dependsOn": [],
							"policy": {
								"secureInput": false
							},
							"userProperties": [],
							"typeProperties": {
								"pipeline": {
									"referenceName": "Child - Pipeline",
									"type": "PipelineReference"
								},
								"waitOnCompletion": true,
								"parameters": {
									"StorageUrl": {
										"value": "@variables('StorageUrl')",
										"type": "Expression"
									}
								}
							}
						}
					]
				}
			}
		],
		"variables": {
			"StorageUrl": {
				"type": "String",
				"defaultValue": "test-url"
			}
		},
		"folder": {
			"name": "Core Code"
		},
		"annotations": []
	}
}

the serialisation and deserialisation process in Save-SynapseObjectAsFile.ps1 means the activities array, and any nested activities, are accidentally converted back into a object as opposed to an array of objects.

{
	"name": "Run all notebooks",
	"properties": {
		"activities": {
		          "name": "ForEach Notebook",
		          "type": "ForEach",
		          "dependsOn": [],
		          "userProperties": [],
		          "typeProperties": {
			  "items": {
				  "value": "<an_array_of_string_values_exposed_via_@item()>",
				  "type": "Expression"
			  },
			"activities": {
			        "name": "Child - Pipeline",
			        "type": "ExecutePipeline",
			        "dependsOn": [],
			        "policy": {
				        "secureInput": false
			        },
			        "userProperties": [],
			        "typeProperties": {
				        "pipeline": {
					        "referenceName": "Child - Pipeline",
					        "type": "PipelineReference"
				        },
				        "waitOnCompletion": true,
				        "parameters": {
					"StorageUrl": {
						"value": "@variables('StorageUrl')",
						    "type": "Expression"
					 }
				    }
			       }
		          }
		     }
		},
		"variables": {
			"StorageUrl": {
				"type": "String",
				"defaultValue": "test-url"
			}
		},
		"folder": {
			"name": "Core Code"
		},
		"annotations": []
	}
}

This fails Synapse's REST-API validations when attempting to deploy. This PR is to start the conversation around addressing it (or fix it altogether).

[NowinskiK]

Thank you, Alex for your contribution. Could you please add a unit test to cover the use case?

[Alex-Swann]

Thank you, Alex for your contribution. Could you please add a unit test to cover the use case?

I hadn't spotted those. Thanks, I spotted an edge case too. Let me know if they align with both the fix and everything else the PS file is responsible for 👍