Allow #[from] to accept backtrace companions

[RAprogramm]

Summary

• relax the derive input validator to tolerate #[backtrace] and optional #[source] companions when #[from] is present
• teach the generated From impls to populate companion backtrace/source fields so the code compiles
• add positive regression coverage for backtrace companions and update trybuild fixtures for the stricter checks

Testing

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

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https://chatgpt.com/codex/tasks/task_e_68ccb6f9a4c4832b8d6d59f0a722f4ba