[typespec-ts] Complete multipart/form-data and multipart content-type support

## Overview

This is an umbrella tracking issue for completing multipart support in the TypeScript emitter (`@azure-tools/typespec-ts`). It consolidates several related issues migrated from [Azure/autorest.typescript](https://github.com/Azure/autorest.typescript), which is being retired in favor of this repo.

Each work item below maps to an original autorest.typescript issue. Each sub-item may still require its own PR.

## Work items

- [ ] **Support other multipart and form content types** — handle `application/x-www-form-urlencoded` and `multipart/mixed` in codegen (currently only `multipart/form-data` is supported).
- Migrated from Azure/autorest.typescript#2277

- [ ] **Fill modular `multipart/form-data` gaps**
- Anonymous models with `@multipartBody` -- resolved
- Text parts with a custom content-type header
- `additionalProperties` support
- Migrated from Azure/autorest.typescript#3001

- [ ] **Fix sample generation for required multipart body** — sample generator emits an empty body (`{}`) instead of the required multipart payload.
- Migrated from Azure/autorest.typescript#3633

- [ ] **Multipart response deserialization** — generate correct deserialization for `multipart/mixed` / `multipart/*` responses instead of calling a model deserializer on the raw body. Blocked by runtime support in Azure/azure-sdk-for-js#37070.
- Migrated from Azure/autorest.typescript#3691

## Notes

- Test coverage (formerly autorest.typescript#3747) is tracked as part of the feature work above, not as a standalone issue.

Start at the TypeScript emitter (@azure-tools/typespec-ts) and choose one unchecked work item rather than treating this umbrella issue as a single change. Review the migrated Azure/autorest.typescript issues and the runtime dependency Azure/azure-sdk-for-js#37070; the selected item is done when its multipart behavior and test coverage are implemented, with response deserialization remaining blocked if applicable.

Written by the indexing model from the issue text.