Handle name collisions for unknown variants

[ghost]

Handle cases where there can be a collision between a name in a service model and the generated Unhandled/Unknown variants in:

• .send() function on fluent builders (triggered by a member named "send")
• Unhandled variant on errors
• Unknown variant on unions
• Unknown variant on enums

[rcoh]

we have a "naming obstacle course model"—we can add all of these cases to it as a long-term regression test