fix(registration): keep stored credentials valid during refresh (backport #977)

[mergify]

see #876

Summary by CodeRabbit

• Bug Fixes
  • Improved credential rotation during registration.
  • Existing credentials remain valid until registration completes successfully.
  • Pending credentials are automatically committed after successful registration.
  • Pending credentials are discarded when registration or server-health checks fail.
  • Authentication now handles credential replacement more reliably without interrupting service.
  • Overlapping registration attempts are now handled safely and consistently.

---

This is an automatic backport of pull request #977 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 71a6f53 has failed:

On branch mergify/bp/cryostat-v4.2/pr-977
Your branch is up to date with 'origin/cryostat-v4.2'.

You are currently cherry-picking commit 71a6f53.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/main/java/io/cryostat/agent/Registration.java

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/main/java/io/cryostat/agent/WebServer.java
	both modified:   src/test/java/io/cryostat/agent/RegistrationTest.java
	both modified:   src/test/java/io/cryostat/agent/WebServerTest.java

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally