Test resharing with a changed reconstruction threshold for Frost, CKD, and DamgardEtAl

[gilcu3]

Background

PR #3640 made per-domain reconstruction thresholds mutable via resharing: run_key_resharing_client now takes both old_threshold and new_threshold, and the resharing computation uses old_threshold for all four providers (eddsa/key_resharing.rs, ckd/key_resharing.rs, ecdsa/key_resharing.rs, robust_ecdsa path). Per the design, a domain's t can only change through resharing.

The only test that actually changes a t during resharing (crates/node/src/tests/reconstruction_thresholds.rs, resharing__should_apply_updated_thresholds_while_preserving_unchanged_ones, t=4→2) covers CaitSith only. Frost, CKD, and DamgardEtAl reshare exclusively with old == new, so a regression in the old/new-threshold plumbing for those protocols would pass CI.

Acceptance Criteria

• Integration coverage for a resharing that changes t for a Frost domain and a CKD domain, asserting the new t gates signing availability afterward (mirroring the existing CaitSith case).
• Coverage for a DamgardEtAl domain reshared with a changed t, asserting the new 2t−1 signer requirement takes effect.
• Each test fails if the per-domain old_threshold/new_threshold value is dropped or swapped for the network-wide threshold.