Missing guidance for middleware registration in E2E tests #3295
Description
Current Problem:
The E2E testing documentation at https://docs.nestjs.com/fundamentals/testing#end-to-end-testing doesn't explain the proper order for middleware registration when using different HTTP adapters.
Missing Information:
- With Express adapter: app.use() can be called before app.init() without errors
- With Fastify adapter: app.init() must be called before app.use() to avoid "this.instance.use is not a function" error
This inconsistency can confuse developers when switching between adapters or writing E2E tests for Fastify-based applications.
Suggested Addition:
Add a section explaining adapter-specific middleware registration patterns in E2E tests, including code examples for both Express and Fastify adapters.