fix(genui): remove duplicate 'component' in required list in prompt

- Use a `Set` to avoid duplicate entries when generating the `required` list for components in the catalog schema.
- Update golden files to reflect the fix.
- Add mock asset handler to `travel_app` tests to fix failing test.

Follow-up for #873.

Author: gspencergoog

packages/genui/lib/src/facade/prompt_builder.dart

@@ -410,10 +410,10 @@ final class _BasicPromptBuilder extends PromptBuilder {
                 'component': {'const': item.name},
                 ...item.dataSchema.value['properties'] as Map<String, dynamic>,
               },
-              'required': [
+              'required': {
                 'component',
                 ...?item.dataSchema.value['required'] as List?,
-              ],
+              }.toList(),
             },
           ],
           'unevaluatedProperties': false,

packages/genui/test/facade/prompt_builder_test.golden/all_operations_with_dataModel_false.txt

@@ -515,7 +515,6 @@ When constructing UI, you must output a VALID A2UI JSON object representing one
             }
           },
           "required": [
-            "component",
             "component",
             "text"
           ]

packages/genui/test/facade/prompt_builder_test.golden/all_operations_with_dataModel_true.txt

@@ -517,7 +517,6 @@ When constructing UI, you must output a VALID A2UI JSON object representing one
             }
           },
           "required": [
-            "component",
             "component",
             "text"
           ]

packages/genui/test/facade/prompt_builder_test.golden/create_and_update_with_dataModel_false.txt

@@ -513,7 +513,6 @@ When constructing UI, you must output a VALID A2UI JSON object representing one
             }
           },
           "required": [
-            "component",
             "component",
             "text"
           ]

packages/genui/test/facade/prompt_builder_test.golden/create_and_update_with_dataModel_true.txt

@@ -515,7 +515,6 @@ When constructing UI, you must output a VALID A2UI JSON object representing one
             }
           },
           "required": [
-            "component",
             "component",
             "text"
           ]

packages/genui/test/facade/prompt_builder_test.golden/create_only_with_dataModel_false.txt

@@ -512,7 +512,6 @@ When constructing UI, you must output a VALID A2UI JSON object representing one
             }
           },
           "required": [
-            "component",
             "component",
             "text"
           ]

packages/genui/test/facade/prompt_builder_test.golden/create_only_with_dataModel_true.txt

@@ -514,7 +514,6 @@ When constructing UI, you must output a VALID A2UI JSON object representing one
             }
           },
           "required": [
-            "component",
             "component",
             "text"
           ]

packages/genui/test/facade/prompt_builder_test.golden/update_only_with_dataModel_false.txt

@@ -505,7 +505,6 @@ When constructing UI, you must output a VALID A2UI JSON object representing one
             }
           },
           "required": [
-            "component",
             "component",
             "text"
           ]

packages/genui/test/facade/prompt_builder_test.golden/update_only_with_dataModel_true.txt

@@ -507,7 +507,6 @@ When constructing UI, you must output a VALID A2UI JSON object representing one
             }
           },
           "required": [
-            "component",
             "component",
             "text"
           ]