Fix no_std doc build regressions

[RAprogramm]

Summary

• import alloc::String for response helpers so no_std builds compile again
• gate the std::io::Error conversion example and pull in ToOwned for AppCode::from_str
• bump masterror to 0.24.6 with changelog and README updates

Testing

• cargo +nightly fmt --
• cargo clippy -- -D warnings
• cargo build --all-targets
• cargo test --all
• cargo test --doc -p masterror --no-default-features
• cargo doc --no-deps
• cargo audit

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https://chatgpt.com/codex/tasks/task_e_68d7d2baaaa0832ba85a27f9b43a8154