Optimize tonic metadata serialization

[RAprogramm]

Summary

• reuse stack-based formatting for gRPC HTTP status and retry metadata in the tonic converter
• add a regression test around Status::from(AppError::timeout(..)) to lock down metadata strings
• bump the crate to 0.24.3 and refresh the changelog and README snippet

Testing

• cargo +nightly fmt --
• cargo +nightly clippy -- -D warnings
• cargo +nightly build --all-targets
• 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_68d746bf4f08832bbcf33fd9d76d2443