security/acme-client: DNS Sleep time reset to 0

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Describe the bug
Using ACME Client (with step-ca,powerdns) after a failed attempt, I set DNS sleep time to 60, but then it gets set back to 0. creating another failed attempt. (also, it leaves stale TXT files in DNS). See attached file for more information.

To Reproduce
Steps to reproduce the behavior:

1. Go to Services > ACME Client > Challenge Types > Add
2. Enter
   Challenge Type: DNS-01
   DNS Service PowerDNS.com
   DNS Sleep Time 0
   URL: PowerDNS Server URL
   Server ID: localhost
   Token: PowerDNS API Token
   Save
3. Go to ACME Client > Certificates > Add
   ...
   ACME Account step-ca
   Challenge Type Power DNS
   ...
   SAVE
4. Issue or renew Certificate
   It fails, because apparently with DNS Sleep Time set to 0, it goes to public DNS servers? (That's a separate issue, I should be able to specify the DNS servers it uses).
5. After it fails, but before the timeout, go back to Challenge Types, Edit, change DNS Sleep Time to 60, then try to renew again, or delete and recreate the entry.
6. Let it fail, go back to Challenge Types, and Sleep Time is set back to 0
7. There also seems to be a timing issue where if I correct everything, make a proper request, but the timeout for the original request hasn't been hit yet, I'll get a certificate issued, according to step-ca, but it ends up showing as failed in the UI
8. Also, it is leaving a ton of TXT entries in my DNS for the failed attempts
9. See error
   Expected behavior
   The DNS Sleep Time should stay at the value set
   A failed attempt shouldn't overwrite the later successful attempt, or it shouldn't let you make a second attempt until the first is completely timed out.
   It should always clean up TXT entries

Screenshots
If applicable, add screenshots to help explain your problem.

Relevant log files
see attached file from my Lab AI

Additional context
see attached

Environment
Version 26.7.3_11
Architecture amd64
os-acme-client (installed) 4.16_1 821KiB 3 OPNsense ACME Client
step-ca: Smallstep CA/0.28.4 (linux/amd64), release date 2025-07-14, image smallstep/step-ca:0.28.4
PowerDNS: PowerDNS Authoritative Server 4.9.17, image powerdns/pdns-auth-49:latest, backend gmysql

20260913-opnsense-acme-clean.md

Start at Services > ACME Client > Challenge Types and reproduce the DNS Sleep Time reset using the PowerDNS and step-ca configuration described. Review the attached log file and trace a failed renewal followed by a second attempt. Done means the configured sleep value persists, overlapping attempts are handled consistently, and failed attempts clean up their TXT entries.