Update GraphQL Emitter to use GraphQL Mutation Engine

[FionaBronwen]

Summary

This PR wires up the mutation engine to the schema emitter, ensuring types are mutated before registration and materialization.

Changes

Schema emitter (src/schema-emitter.ts):

• Initialize GraphQLMutationEngine in constructor
• enum handler: mutates enum before registering
• model handler: mutates model before registering
• exitEnum/exitModel: uses cached mutated names for materialization

Flow

Original TSP
↓
Mutation Engine
↓
Mutated TSP
↓
GraphQL Type Registration/ Materialization
↓
GraphQL Schema

Example

// TypeSpec input
model $Model { 
  field: string 
}

// After mutation
model _Model { 
  field: string 
}

// GraphQL output
type _Model {
  field: String
}

[steverice]

I'm confused as to what the point of the engine is if we have to enumerate its mutations explicitly. Can you explain?