feat: Implements cf accessor on Request

[dom96]

This accessor is missing, so users would instead need to use request.js_object.cf manually to access it, which isn't ideal.

[ask-bonk]

The code looks clean. Let me review the overall PR:

1. Implementation (_workers.py): The cf property is clean and follows the existing pattern (same as body using _jsnull_to_none). The type annotation JsProxy | None is correct. The docstring is clear and includes a link to the relevant documentation.

2. Tests (test_sdk.py): The test covers both cases — a newly constructed Request without cf (returns None) and an incoming request through workerd where cf is present. The JsProxy import was already present in the file.

3. Placement: The property is placed between body_used and cache, which is reasonable alphabetical ordering.

There are no logic bugs, security issues, backward compatibility concerns, or other actionable problems. The change is a straightforward addition of a missing accessor property that follows existing patterns in the codebase.

LGTM

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