fix(controller): Do not extend sandbox ShutdownTime on malformed pause-retention annotation

[vishalmore90]

Ⅰ. Describe what this PR does

Fixes a logic bug where a malformed agents.kruise.io/reserve-paused-sandbox-duration annotation causes resolveRetentionAnnotationOrDefault to silently return managed=true.

This previously caused handlePauseTimeout to extend Spec.ShutdownTime by ~100 years the next time the sandbox auto-pauses, instead of cleanly treating the invalid annotation as absent.

The fix correctly returns managed=false for unparsable annotations and emits a Kubernetes Warning event to help operators identify the misconfiguration.

Ⅱ. Does this pull request fix one issue?

fixes #889

Ⅲ. Describe how to verify it

1. Create a Sandbox with a PauseTime in the near future and a ShutdownTime.
2. Annotate the sandbox: kubectl annotate sandbox <name> agents.kruise.io/reserve-paused-sandbox-duration=invalid-duration.
3. Wait for PauseTime to be reached.
4. Observe that a Warning event is generated (InvalidRetentionAnnotation).
5. Observe that the Sandbox reaches its PauseTime and gets paused, but its ShutdownTime remains unchanged and is not extended by 100 years.
6. Verify the Sandbox gets correctly garbage collected when its original ShutdownTime is reached.
7. Run make test and ensure the updated test case invalid annotation patches paused without extending shutdown time passes.

Ⅳ. Special notes for reviews

The test case "invalid annotation patches paused without backfilling default" in sandbox_controller_test.go inadvertently encoded the bug's behavior (expectShutdownChange: true). This PR updates that test case to correctly expect expectShutdownChange: false and renames it.

[kruise-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.39%. Comparing base (825de1d) to head (01da012).
⚠️ Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #890      +/-   ##
==========================================
+ Coverage   82.31%   82.39%   +0.07%     
==========================================
  Files         244      254      +10     
  Lines       19782    21461    +1679     
==========================================
+ Hits        16284    17683    +1399     
- Misses       2888     3079     +191     
- Partials      610      699      +89     

Flag	Coverage Δ
unittests	82.39% <100.00%> (+0.07%)	⬆️

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[vishalmore90]

/Review