Fix: Make Response body contract explicitly nullable

[techmahedy]

The Response body contract now matches Doppar’s real runtime behavior.

What changed

1. Updated Response::getBody() to return ?string
2. Updated the Response facade annotation to reflect the nullable return type
3. Added regression coverage for response preparation paths that intentionally clear the body

Why

Doppar already allows null response bodies:

Tests

Added focused response tests to verify:

1. HEAD requests clear the body during prepare()
2. empty responses clear the body during prepare()