Chain-compatible presignature selection for verify-foreign-tx

[anodar]

Background

verify-foreign-tx provider gates requests on the available-chains watch channel, but the leader still acquires an arbitrary presignature via take_owned.

Presignature's co-owners may not all support the requested chain: a non-supporting follower rejects the attempt against its supporters snapshot, the signing round fails, and the presignature is wasted.

The leader must instead select a presignature co-owned exclusively by participants that support the request's chain.

User Story

No response

Acceptance Criteria

• The verify-foreign-tx leader only takes presignatures whose participants all support the requested chain (alive participants intersected with the supporters map from Publish foreign-chain supporters from the indexer via a watch channel #3852).
• A request for a chain whose supporter set cannot cover any presignature does not consume presignatures.

Resources & Additional Notes

No response