Profile.Defaults should not allow Name or Source to be defined

[carlos-zamora]

Found as a part of this investigation: https://github.com/microsoft/terminal/pull/19156/files#r2248959745

We should probably not let users do this as it lets them apply the same name and source across all their profiles unless specifically overridden.

The fix would be pretty simple. Just add a ClearName() and ClearSource() here:

terminal/src/cascadia/TerminalSettingsModel/CascadiaSettingsSerialization.cpp

Lines 807 to 811 in a5d916f

	settings.baseLayerProfile = Profile::FromJson(json.profileDefaults);
	// Remove the `guid` member from the default settings.
	// That will hyper-explode, so just don't let them do that.
	settings.baseLayerProfile->ClearGuid();
	settings.baseLayerProfile->Origin(OriginTag::ProfilesDefaults);