Summary

• add a Context::redact_field_mut helper for in-place metadata policy updates and reuse the shared setter logic
• extend response tests to cover JSON serialization redaction for ErrorResponse and ProblemJson, including metadata hashing and redaction behaviour
• bump crate version to 0.20.0 with changelog/readme updates and add serde-based regression tests for opt-in internal formatters

Testing

• cargo +nightly fmt --
• cargo +1.90.0 clippy -- -D warnings
• cargo +1.90.0 build --all-targets
• cargo +1.90.0 test --all
• cargo +1.90.0 doc --no-deps
• cargo +1.90.0 audit
• cargo +1.90.0 deny check

https://chatgpt.com/codex/tasks/task_e_68d3420edf4c832ba0b1b5ecc6bced63