chore: job restart not needed when governance threshold changes

[gilcu3]

Closes #3838

[claude]

Pull request overview

Drops the threshold field from the restart-triggering criteria in participants_change_requires_restart, so a governance-threshold-only change is now absorbed live rather than tearing down the running job. Completes the follow-up planned in #3838: now that per-domain reconstruction thresholds drive the protocol (#3640), the network-wide governance threshold no longer affects a running job. The doc comment and the matching unit test are updated to reflect the new behavior.

Changes:

• Remove the old.threshold != new.threshold branch (and its TODO(#3838)) from participants_change_requires_restart in crates/node/src/coordinator.rs.
• Update the function-level doc comment to explain why a governance-threshold-only change no longer forces a restart.
• Rename and flip the corresponding unit test to assert that a governance-threshold-only change does not require a restart.

Reviewed changes

Per-file summary

File	Description
crates/node/src/coordinator.rs	Drops threshold-based restart branch; updates doc comment; flips the threshold-change unit test to assert no restart.

Findings

Verified via grep that within crates/node/src, ParticipantsConfig::threshold is read in exactly one non-test production location — coordinator.rs:569 where it feeds sender.wait_for_ready(...) as a one-shot pre-flight gate at job startup. All the ongoing protocol paths (triple gen at providers/ecdsa/triple.rs:127, robust presign at providers/robust_ecdsa/presign.rs:57, ECDSA/EdDSA/robust sign paths) use only the participant list from mpc_config.participants.participants and the per-domain reconstruction_threshold. So the doc comment's claim ("per-domain reconstruction thresholds drive the running protocol") holds, and a live threshold change is safe to absorb — a new value only becomes effective at the next natural restart (epoch change, membership change, own-port change, TLS-key rotation, etc.), which is the intended outcome. The contract itself validates threshold <= n on any proposed governance change, so there is no risk of an unsatisfiable threshold sneaking in.

No blocking or non-blocking issues found. Small, well-scoped follow-up that closes #3838 cleanly.

✅ Approved