Exercise 1: A CORS-Header Middleware — Possible Solution
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THE MIDDLEWARE
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    def cors_middleware(request, next):
        response = next(request)
        response.headers["Access-Control-Allow-Origin"] = "*"
        return response

This follows the exact same "after next() returns" shape as the
chapter's own timing_middleware -- the header is set on the real
Response object only once the handler (and everything nested inside
it) has already finished producing it.

TESTING IT
------------------------------
    def handler1(request):
        return Response("hello")

    app1 = build_chain([cors_middleware], handler1)
    result1 = app1({"path": "/"})

    print(result1.body)      # hello
    print(result1.headers)   # {'Access-Control-Allow-Origin': '*'}

RESULT, VERIFIED DIRECTLY
------------------------------
    body: hello
    headers: {'Access-Control-Allow-Origin': '*'}

The real Response object returned by the whole chain carries both the
handler's own real body ("hello") and the header the middleware added
afterward, confirming the middleware genuinely modified the same
object the handler produced rather than creating a separate one.

WHY THIS WORKS AS AN ANSWER
----------------------------
It follows the chapter's own established "run code after next()
returns, then modify the response before handing it back" pattern
exactly, and verifies the result against a real Response object's own
headers dict rather than only asserting the header would be present.