Cover late-join multisig registry recovery

[kentbull]

References WebOfTrust/keria#316.

This adds an integration test for the late-join multisig registry gap from KERIA #316. The test creates an A/B multisig registry, issues a credential to make registry/TEL CESR export available through credential export, rotates C into the group, confirms C does not initially list the registry, imports the credential CESR, renames the registry, and verifies C can read/export the credential.

It also verifies the recovered registry remains usable by issuing a new credential with A, B, and C after the join. The shared multisig credential helper now returns the issued credential SAID so tests can track the exact request without local wrappers.

Validation:

• npm run build
• Targeted SignifyTS integration: multisig-late-join-registry-repro.test.ts, 1 passed
• git diff --check

[kentbull]

Related PRs for the same WebOfTrust/keria#316 late-join multisig registry work:

• Drain imported CESR before registry rename keria#439
• Reproduce late-join multisig registry gap signifypy#149

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.53%. Comparing base (f9b223d) to head (c24d45e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #388   +/-   ##
=======================================
  Coverage   97.53%   97.53%           
=======================================
  Files           1        1           
  Lines         325      325           
  Branches       24       24           
=======================================
  Hits          317      317           
  Misses          8        8           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.