[Bug]: TCGC's body parameter should have serialization option

### Describe the bug

See playground.

### Reproduction

https://azure.github.io/typespec-azure/playground/?e=%40azure-tools%2Ftypespec-client-generator-core&c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7DQp1c2luZyBIdHRwOw0KDQpAc2VydmljZQ0KbmFtZXNwYWNlIFRlc3TGHWRvYygiT3V0cHV0IG1vZGVsIHdpdGggdmlzaWJpbGl0eSBwcm9wZXJ0aWVzLiIpDQrGJVbJIE3FEHsNCiAgxk1SZXF1aXJlZCBzdHJpbmcsIGlsbHVzdHJhdOQAl2EgcmVhZG9ubMlZecVXICBAynQoTGlmZWN5Y2xlLlJlYWTFH8Q3UHJvcDrHWOUAvyAgI3N1cHByZXNz8AEKL21ldGFkYXRhLWlnbm9yZWQiICJGb3IgdGVzdCLzAK5pbnQzMvEArXF1ZXL%2FAKroAKpRxCvGIMU2xArFCeYAtsVg5wC1z3vmANFbXdF%2BY3JlYXRl33%2FHf0PFLMUhxjfsAS1bXdZ65QCV03l1cGTfecp5VcUsxSHGN%2BsA8Nh4Ym9vbNF1ZGVsZd91yXVExSzFIcY3xnXEWGVhbs11xBrEUCB0aGF0IGRvZXMgbm90IGV4aXN0IGluIGFueSBwYXlsb2FkyHNpbsV1bGXKcsUZbm9ux2kixAvkA219xGtAZ2V0DQpvcCBnZXTlAxAoQGJvZHlSb2%2FEYnB1dDrwAzEpOucDM8QoxAlv5QOD0SjEYuUAzm9wIHBvc3TSWeQDy9ZTxAjLICzIS%2BYDwskYxjbkAXboAi7EKg%3D%3D&options=%7B%7D&vs=%7B%7D

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/azure/typespec-azure/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that this issue is about the Azure libraries for typespec. For bug in the typespec language or core libraries file it in the [TypeSpec repo](https://github.com/Microsoft/TypeSpec/issues/new/choose)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.