feature(sandbox-manager): refine paused retention timeout handling

[AiRanthem]

Summary

• Compute create-time auto-pause retention from the persisted reserve-paused value instead of storing a parsed duration on request extensions.
• Split timeout math into a pure helper so resume placeholder calculation has no annotation side effects.
• Update E2B tests around resume placeholder/default retention behavior.

Test Plan

• GOCACHE=/private/tmp/go-build-cache go test ./pkg/pausedretention
• GOCACHE=/private/tmp/go-build-cache go test ./pkg/controller/sandbox -run 'TestSandboxReconciler_(HandleShutdownTimeout|HandlePauseTimeout|AutoPauseReservePausedRetention)'\n- GOCACHE=/private/tmp/go-build-cache go test ./pkg/servers/e2b/models\n- GOCACHE=/private/tmp/go-build-cache go test ./pkg/servers/e2b -run 'Test.*(Resume|Pause|Timeout|ReservePaused)'\n- git diff --check

[kruise-bot]

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[codecov]

Codecov Report

❌ Patch coverage is 97.35450% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.97%. Comparing base (ce4b897) to head (4cfd619).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/controller/sandbox/sandbox_controller.go	96.61%	2 Missing ⚠️
pkg/pausedretention/retention.go	90.47%	1 Missing and 1 partial ⚠️
pkg/servers/e2b/timeout.go	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #566      +/-   ##
==========================================
+ Coverage   79.71%   79.97%   +0.25%     
==========================================
  Files         195      203       +8     
  Lines       14107    14814     +707     
==========================================
+ Hits        11246    11848     +602     
- Misses       2457     2541      +84     
- Partials      404      425      +21     

Flag	Coverage Δ
unittests	79.97% <97.35%> (+0.25%)	⬆️

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