fix(request): split charge response errors by failure layer

[betterclever]

Summary

Fixes #453.

• add typed PaymentError variants for charge-response failures after credential submission
• keep 402 responses classified as PaymentRejected, matching MPP credential rejection semantics
• classify non-402 4xx responses as application-layer payment response errors, and 5xx responses as server-layer payment response errors

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-features --locked
• RUSTDOCFLAGS='-D warnings' cargo doc --workspace --all-features --no-deps --locked
• typos

Note: local make check could not run literally because the Homebrew cargo in this environment does not support +nightly; the equivalent stable-toolchain fmt/clippy/test/doc gates above passed. cargo deny check currently fails on existing transitive quick-xml advisories through toon-format/syntect, unrelated to this PR.