Check tx index state when trade lookups are empty

[alastairong1]

Summary

• keep the existing direct transaction trade lookup in implement-trades-tx-hash
• add an explicit transaction index-state check when that lookup returns no trades
• return 202 not yet indexed only when the transaction is still missing from the index, otherwise keep 404 for indexed transactions with no trades

Why

The base branch already uses a direct transaction lookup, so the full lookup speedup from albionlabs/albion.rest.api is not needed here. The remaining gap is correctness: the route documents a 202 response for not-yet-indexed transactions, but an empty direct lookup currently falls through to 404.

Validation

• local cargo fmt ran successfully
• full cargo test/cargo check could not be completed in this clone because the nested rain.orderbook workspace is missing generated contract artifacts (out/...json) required by its bindings build

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[alastairong1]

PR #50 already handles NotYetIndexed/202 via RaindexError::TradesIndexingTimeout mapping. Closing in favor of that approach.