[fix][evaluation] await sandbox cleanup before experiment deletion

[bitholic]

Deleting a SandboxAgent experiment could remove the experiment before its existing sandbox cleanup completed. Single and batch deletion now resolve the existing incomplete-item cleanup references and wait for the existing Destroy retry result before deleting. A lookup or cleanup failure returns an error and leaves the experiment undeleted.

The wait path reuses the existing ten-second retry budget. Existing non-deletion callers retain the default asynchronous cleanup behavior; the source-space routing and authorization remain unchanged.

Validation:

• 57 related test/subtest passes across 14 top-level race-tested cases, with no failures or skips.
• Combined compilation passed for the downstream agent-studio/evaluation modules and both service entrypoints using the isolated core changes.

[codecov]

Codecov Report

❌ Patch coverage is 83.56164% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...d/modules/evaluation/domain/service/target_impl.go	82.97%	5 Missing and 3 partials ⚠️
...ation/domain/service/expt_manage_execution_impl.go	75.00%	1 Missing and 1 partial ⚠️
...ules/evaluation/domain/service/expt_manage_impl.go	88.88%	1 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #690      +/-   ##
==========================================
- Coverage   78.92%   78.91%   -0.01%     
==========================================
  Files         707      707              
  Lines       88041    88086      +45     
==========================================
+ Hits        69484    69515      +31     
- Misses      14575    14583       +8     
- Partials     3982     3988       +6     

Flag	Coverage Δ
unittests	78.91% <83.56%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ation/domain/service/expt_manage_execution_impl.go	83.90% <75.00%> (-0.15%)	⬇️
...ules/evaluation/domain/service/expt_manage_impl.go	66.94% <88.88%> (+0.37%)	⬆️
...d/modules/evaluation/domain/service/target_impl.go	84.81% <82.97%> (-0.46%)	⬇️

... and 2 files with indirect coverage changes

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