Restore integrations and prepare 0.20.3

[RAprogramm]

Summary

• restore the axum integration module and update response tests to assert over serde_json::Value
• harden converter metadata for redis/reqwest/sqlx/tonic and switch ProblemJson metadata storage to Cow
• bump the crate to version 0.20.3 with changelog and README updates

Testing

• cargo +nightly fmt --
• cargo +1.90.0 build --all-targets
• cargo +1.90.0 clippy -- -D warnings
• cargo +1.90.0 test --all
• cargo +1.90.0 doc --no-deps
• cargo audit --color never
• cargo deny check

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https://chatgpt.com/codex/tasks/task_e_68d3722b95b0832b826449da70bc7cb1