redistribution: refund_identity field

[dvush]

📝 Summary

Adds refund identity

💡 Motivation and Context

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✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[Copilot]

Pull Request Overview

This PR adds a new optional field refund_identity to the Metadata struct, enabling the redistribution mechanism to optionally return a refund identity.

• Introduces the refund_identity field in Metadata with a default value of None.
• Updates order redistribution logic to use refund_identity when present.
• Adjusts synthetic order initialization to include the refund_identity field.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/rbuilder/src/primitives/mod.rs	Added refund_identity field and initialized it in with_current_received_at().
crates/rbuilder/src/backtest/redistribute/mod.rs	Updated order_redistribution_address() to check refund_identity and return it if available.
crates/rbuilder/src/backtest/build_block/synthetic_orders.rs	Initialized refund_identity as None in synthetic order metadata.