Finalize context error migration

[RAprogramm]

Summary

• refresh the integration converters to build structured Context metadata with retry hints and redaction before materializing Error values
• add the AppCode::UserAlreadyExists classification, wire it into the RFC7807 mappings, and extend the SQLx/reqwest tests to cover the new metadata expectations
• update the trybuild UI snapshots and bump the workspace docs/version to 0.18.0 with an entry in the changelog

Testing

• TRYBUILD=overwrite cargo +1.90.0 test --test error_derive_from_trybuild
• 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 audit
• cargo deny check

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https://chatgpt.com/codex/tasks/task_e_68d270f1f514832bae6a9eea3af9f605