fix: align PaymentBodyLayer retry and cache semantics

[vikions]

Summary

• return a fresh body-bound WWW-Authenticate challenge with Cache-Control: no-store after failed payment verification
• mark successful body-aware receipt responses as private while preserving existing cache directives
• add regression tests for both retryable 402 responses and receipt cache behavior

Motivation

PaymentBodyService diverged from the standard PaymentService behavior in two protocol-relevant paths:

• failed verification returned a bare 402 without a fresh payment challenge
• successful responses containing Payment-Receipt did not receive private cache semantics

Body-digest binding should not change the retry lifecycle or receipt caching behavior of the Tower payment middleware.

Testing

• cargo test --lib body_service --no-default-features --features tower
• cargo fmt --check
• cargo clippy --lib --no-default-features --features tower
• git diff --check

All focused tests pass. Clippy reports only pre-existing cfg-dependent warnings unrelated to this change.