Avoid allocations by reusing AppErrorKind labels

[RAprogramm]

Summary

• add an AppErrorKind::label helper reused by the Display impl and downstream formatters
• return borrowed labels from Error::render_message, ProblemJson fallbacks, and ErrorResponse mappings to avoid extra String creation
• update response tests to assert borrowed fallbacks and preserve redactable behaviour

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 deny check
• cargo audit

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https://chatgpt.com/codex/tasks/task_e_68d72c231130832bbaa16dda1c1fecd2