Add formatter coverage tests and bump version

[RAprogramm]

Summary

• add runtime coverage for every formatter variant generated by the Error derive, including lowercase/uppercase assertions
• extend the trybuild suite with formatter pass/fail cases that cover supported and unsupported specifiers
• bump the crate version to 0.5.6 and document the change in the changelog and README

Testing

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

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https://chatgpt.com/codex/tasks/task_e_68cce96f90f0832b9ee14e16d1ba2efe