[graphql] Add ./mutation-engine subpath export for standalone mutation pipeline usage (#11565)

## Summary

Add a `./mutation-engine` subpath export to `@typespec/graphql` to
enable consumers to use the GraphQL mutation pipeline independently of
the emitter. This allows for other future GraphQL related packages to
use the GraphQL mutation engine to transform the input TSP into GraphQL
compatible shapes in using the same library, reducing duplicate
implementations.

The subpath exports:
- `mutateSchema`, `createGraphQLMutationEngine` — orchestration
- `resolveTypeUsage` — type usage analysis (input vs output)
- `GraphQLTypeContext`, `TypeGraph` — types
- Mutation classes (`GraphQLModelMutation`, etc.)

## Test plan

- [x] Existing tests pass (368 tests)
- [x] Package builds successfully
- [x] Subpath resolves correctly

---------

Co-authored-by: Timothee Guerin <timothee.guerin@outlook.com>

Author: FionaBronwen

.chronus/changes/graphql-mutation-engine-exports-2026-7-5-18-8-56.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/graphql"
+---
+
+Add `./mutation-engine` subpath export for standalone mutation pipeline usage

packages/graphql/package.json

@@ -24,6 +24,10 @@
       "typespec": "./lib/main.tsp",
       "types": "./dist/src/index.d.ts",
       "default": "./dist/src/index.js"
+    },
+    "./mutation-engine": {
+      "types": "./dist/src/mutation-engine/index.d.ts",
+      "default": "./dist/src/mutation-engine/index.js"
     }
   },
   "engines": {

packages/graphql/src/mutation-engine/index.ts

@@ -1,3 +1,4 @@
+export { resolveTypeUsage, type TypeUsageResolver } from "../type-usage.js";
 export { GraphQLMutationEngine, createGraphQLMutationEngine } from "./engine.js";
 export {
   GraphQLEnumMemberMutation,