Drain imported CESR before registry rename

[kentbull]

References #316.

This drains any CESR bytes queued by an agent import before the registry rename endpoint aliases an imported registry as local. In the late-join multisig flow, the credential CESR import can queue TEL material and the immediate registry rename can otherwise cause the queued issue event to be parsed under local registry handling.

The change keeps the import/rename ordering deterministic without changing the public API.

[kentbull]

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

• Reproduce late-join multisig registry gap signifypy#149
• Cover late-join multisig registry recovery signify-ts#388

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.72%. Comparing base (93f6fb2) to head (9181ba8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/keria/app/credentialing.py	50.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (50.00%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #439      +/-   ##
==========================================
- Coverage   87.74%   87.72%   -0.02%     
==========================================
  Files          26       26              
  Lines        5816     5818       +2     
==========================================
+ Hits         5103     5104       +1     
- Misses        713      714       +1     

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[iFergal]

This is only really possible in a script race condition, and the registry rename trick is a workaround. imo we should just add a delay in the script and keep this endpoint clean.

Hopefully, moving to registrars will help resolve this issue in a more permanent way that also works outside scripts.