Support fmt handlers and update diagnostics

[RAprogramm]

Summary

• allow structs and enum variants to call #[error(fmt = ...)] handlers with borrowed fields and the formatter
• drop duplicate missing-attribute diagnostics and refresh the related trybuild expectations
• add a formatter-path success fixture and bump the crate metadata/documentation

Testing

• 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_68cdf7c1180c832ba0fa8e34f4e947df