mmv1/list: Support default_value for optional list scope properties

[BBBmau]

When a url_param_only parameter in a resource's collection URL is optional but has a default_value, the list resource machinery was previously treating an empty/omitted value as a blank string, resulting in malformed URLs like collections//engines/....

Fix this in three places:

• list_resource.go.tmpl: fall back to the field's default_value when the user does not supply a value for an optional scope property
• list_resource_method.go.tmpl: same fallback when setting the property on the temporary ResourceData used to build the list URL
• query_test_file.go.tmpl: include optional fields that have a default_value in the generated list query test (capture + variable declaration + config) so the default is propagated to the list block

This unblocks generate_list_resource: true for resources like google_discovery_engine_control and google_discovery_engine_widget_config whose collectionId parameter defaults to "default_collection".

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit 1069817:

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