Doesn't create models/refs for HttpPart properties when using overloads

Doesn't properly model out the contents of the MutliPart model

[Playground Link](https://typespec.io/playground/?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7DQoNCnVzaW5nIFR5cGVTcGVjLkh0dHDFGEBzZXJ2aWNlDQpuYW1lc3BhY2UgQUkuQ2hhdMUgbW9kZWwgQ29udGV4dFByb3Agew0KICBjxhE%2FOiBSZWNvcmQ8dW5rbm93bj47DQp9yjhTZXNzaW9uU3RhdGXKPXPLFj86IMc7yDplbnVtIEFJ5ACCUm9sZcYzYXNzaXN0YW50LMQOdXNlcsUJc3lzdGVtLM10xjtNZXNzYWfHPnJvbGU6y1Q76QDDbnQ6IHN0cmluZ8UULi4u6wDr7gDMzVhEZWx0YcpdP9heP99f0F9Db21wbGV0aW9uxl1txm%2FIX8d%2B1k3FE%2FABct5l6wDHQGhlYWRlcusAvOQCLTogImFwcGxpY2HELy9qc29ubOYCUSAgZMQ97wCaxVH%2FAJ%2F%2FAJ90RXJyb3JSZXNwb25z5wHBZcQTyGvFINg4xjBjb2Rl7QF66QFIyRTRP0Zp6AJt7QEAy0VkYeQA8GJ5dGVz1EHqAVdSZXF1ZXN07QHDc%2B8BKltd%2FwEn%2FwEn0m9NdWx0aXBhcsd45AGsOiDkA%2BlQYXJ0PNc1PuUDSGZpbOQAoc8t5AESPltdx3xhbGlhcyDoAZg8VD4gPSBUIHz0AbXlAiFvcCBnZctxKOcCYv8CXyIgfCAibegAyy9mb3JtLeQBeSIsIGJvZHnoAUbyAW3oAIr6ARgp5ASu5wC%2F0CzkBLkNCkBvdmVybG9hZCjtALspDQpAcG9zdPIA1UpTT07%2FANnpANksIEDkAML%2BAMf%2FAKT%2FAKT1AKTpAQD3AKn2AW1AyRdC%2FwC1%2FwFi7wC%2B5gCkQHJvdXRlKCIvc3RyZWFtIinoALdTxRBlZP8COP8COP8COP8COP8A1s0s5QTx8wGZ8gDI6gGh%2FwD9zDX%2FAmD%2FAmD%2FAmD%2FAMz%2FAMz%2FAMzSNf8CiP8CiP8CiP8BsvIA5g%3D%3D&e=%40typespec%2Fopenapi3&options=%7B%7D)