Fix bug in offer implementation

[Rigidity]

Note

High Risk
Changes offer-taking coin selection and signing behavior in wallet code paths that move funds; mistakes could enable double-spend or signing attacker-injected spends.

Overview
Fixes take-offer coin selection and signing so the taker cannot reuse or sign coins already spent in the incoming offer bundle.

select_spends now delegates to select_spends_excluding, which treats offer input coin IDs as unavailable when auto-selecting XCH/CAT for the counterparty payment. take_offer passes every coin ID from the offer’s coin_spends into that exclusion list, then returns a TakenOffer (offer + unsigned taker spend_bundle) instead of a finished bundle.

The API take_offer path signs only the taker’s spends (sign_transaction with partial: false) and applies offer.take afterward. Tests use a shared sign_taken_offer helper and add coverage for re-taking your own offer (coin selection error) and for injected malicious spends in the offer (excluded from signing; resulting bundle fails simulation).

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