Move model JSON usage setting out of the emitter

[MaiLinhP]

Addresses #1232

With the generator update to add InputModelType Update function, we can now move model JSON usage setting from the emitter to visitor. This ensures consistency of keeping all OpenAI customization in visitors.

[Copilot]

Pull request overview

Moves the “multipart form models should also be treated as JSON models” customization from the TypeSpec emitter into the C# generator visitor layer, aligning with the plugin’s pattern of keeping OpenAI-specific generation behavior in visitors.

Changes:

• Update ModelSerializationVisitor to set InputModelTypeUsage.Json on multipart-form models via InputModelType.Update(...).
• Remove the emitter-side CodeModel mutation callback that previously OR’ed UsageFlags.Json onto multipart-form models.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
codegen/generator/src/Visitors/ModelSerializationVisitor.cs	Adds visitor-side usage update so multipart form models are treated as JSON for MRW/serialization generation.
codegen/emitter/src/emitter.ts	Removes emitter-time model usage mutation callback; now delegates behavior to the generator visitors.

Comments suppressed due to low confidence (1)

codegen/emitter/src/emitter.ts:7

• CodeModel is no longer referenced after removing the emitCodeModel callback, leaving an unused import that can break TypeScript builds when unused imports are treated as errors.

import {
    emitCodeModel,
    CodeModel,
    CSharpEmitterOptions
} from "@typespec/http-client-csharp";