docs(hmac-auth): include Digest header in signed headers for body validation example

[moying2026]

Fixes #13395\n\nThis PR improves the body validation example in the hmac-auth documentation. Currently, the example computes and sends a Digest header but does not include it in the signed headers list, which could mislead readers about how to properly bind the request body to the HMAC signature.\n\nChanges:\n- Add a note clarifying that validate_request_body alone does not bind the Digest header to the signature\n- Update the Python example to compute the body digest before building the signing string\n- Include the Digest header in both the signing string and the headers list in Authorization\n- Adjust the signing string construction order to match the updated headers list\n\nThis is a docs-only change with no code modifications.