Add GraphQL mutation engine for TSP type transformations

[FionaBronwen]

Summary

This PR introduces a GraphQL mutation engine that leverages the @typespec/mutator-framework to transform TypeSpec types for GraphQL schema generation. This PR establishes the foundation with name sanitization, but the architecture will support additional transformations (e.g., input/output type splitting, visibility filtering) in future PRs.

What it does

The mutation engine wraps the mutator-framework's MutationEngine with GraphQL-specific mutation classes. These classes intercept type graph traversal to apply GraphQL-specific transformations.

Current transformation: Name sanitization ensures all type, field, and member names are valid GraphQL identifiers (e.g., $Invalid$ → _Invalid_, my-field → my_field).

Future transformations (not in this PR):

• Input/output type splitting based on usage
• Custom scalar mapping
• and more :)

Architecture

Class	Purpose
GraphQLMutationEngine	Core engine wrapping MutationEngine
GraphQLEnumMutation	Transforms enum types
GraphQLEnumMemberMutation	Transforms enum members
GraphQLModelMutation	Transforms model types
GraphQLModelPropertyMutation	Transforms model properties
GraphQLOperationMutation	Transforms operations
GraphQLScalarMutation	Transforms scalar types

Changes

New Files:

• src/mutation-engine/engine.ts - Core engine wrapping MutationEngine
• src/mutation-engine/options.ts - GraphQL-specific mutation options
• src/mutation-engine/mutations/*.ts - 6 mutation classes for different type kinds
• test/mutation-engine/graphql-mutation-engine.test.ts - unit tests

Usage

import { createGraphQLMutationEngine } from "@typespec/graphql";

const engine = createGraphQLMutationEngine(program, namespace);

const enumMutation = engine.mutateEnum(myEnum);
const modelMutation = engine.mutateModel(myModel);

// Access the transformed type
console.log(enumMutation.mutatedType.name);

[steverice]

Presumably this should be workspace:~ as well?

[steverice]

As I understand the implementation of the mutation framework, a MutationEngine should capture the mutations for one type of transformation; i.e., we should have a specific GraphQLNamingMutationEngine, and other engines that capture the various different transformations we want to run, rather than a global "GraphQL" engine.

I'm using the HttpCanonicalizer engine as the reference point here — it implements a very specific type of transformation rather than "HTTP" transformations broadly.

It also seems that SimpleMutationEngine might be sufficient for what we need here.