feat(aionrs): map public error codes

[TCP404]

Summary

• add a temporary wire-compatible mirror for the new aionrs public error contract
• map aionrs public error codes to AgentSendError without message-text classification
• keep the legacy aionrs_engine_error_to_send_error entry as a structured compatibility adapter

Verification

• just push
  • migration immutability check
  • cargo fix --allow-dirty --allow-staged
  • cargo clippy --fix --workspace --allow-dirty --allow-staged -- -D warnings
  • cargo fmt --all
  • cargo nextest run --workspace
  • 6333 tests run: 6333 passed, 18 skipped

Notes

• This branch intentionally does not update the aionrs dependency. It mirrors the wire contract while aioncore is still on aionrs v0.1.30.
• After the aionrs public error PR/dependency update lands, replace the private mirror with aion_protocol::events::PublicError.
• Depends conceptually on feat(error): add typed public error contract aionrs#140.