docs: fix undefined request variable in A/B testing middleware example

[DearAsutosh]

Summary

Fixes an undefined variable in the "A/B testing with middleware" example.

The snippet previously referenced:

request.headers.get("cookie")

However, request is not defined in this context. Cloudflare Pages Functions middleware exposes the request via context.request.

This PR updates the example to:

context.request.headers.get("cookie")

so that the code aligns with the Pages Functions API and works as expected.

Fixes #28447

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