Guard std-dependent docs for no_std doc tests

[RAprogramm]

Summary

• gate documentation examples that require the standard library so doctests pass without default features
• switch the response retry example to core::time::Duration to avoid a std dependency
• bump the crate to version 0.24.12, update the changelog, and refresh README guidance

Testing

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

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https://chatgpt.com/codex/tasks/task_e_68db89175e6c832b8576e60b256e8b3f