fix: await background load futures during cancellation and fix loadingTimeoutMs default

[sparknack]

Summary

• Await background load futures in GroupChunkTranslator and ManifestGroupTranslator cancellation cleanup paths to prevent use-after-free / dangling futures
• Add CheckCancellation calls in LoadCellBatchAsync loop for prompt cancellation
• Fix tieredStorage.loadingTimeoutMs default from 0 (immediate failure) to -1 (no timeout)

issue: #48854

Test plan

• Added unit test CancellationStopsMidBatchPush verifying mid-batch cancellation stops loading and raises FollyCancel
• CI passes

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: sparknack
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Details

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[sre-ci-robot]

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✅ CI Loop Results a29ee05

Stage	Result	Duration	Tests
✅ Build	SUCCESS	13.2min	-

Total: 18min | Pipeline | Artifacts

[codecov]

Codecov Report

❌ Patch coverage is 67.50000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.49%. Comparing base (91cf2cc) to head (a29ee05).
⚠️ Report is 7 commits behind head on master.

⚠️ Current head a29ee05 differs from pull request most recent head 339021d

Please upload reports for the commit 339021d to get more accurate results.

Files with missing lines	Patch %	Lines
...gcore/storagev2translator/GroupChunkTranslator.cpp	0.00%	6 Missing ⚠️
...re/storagev2translator/ManifestGroupTranslator.cpp	0.00%	6 Missing ⚠️
internal/core/src/segcore/MemoryPlannerTest.cpp	96.15%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (67.50%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #48855      +/-   ##
==========================================
- Coverage   84.29%   83.49%   -0.81%     
==========================================
  Files         633      645      +12     
  Lines       99080   101186    +2106     
==========================================
+ Hits        83516    84481     +965     
- Misses      15564    16653    +1089     
- Partials        0       52      +52     

Components	Coverage Δ
Client	∅ <ø> (∅)
Core	84.28% <67.50%> (-0.01%)	⬇️
Go	∅ <ø> (∅)

Files with missing lines	Coverage Δ
internal/core/src/segcore/memory_planner.cpp	95.65% <100.00%> (+0.04%)	⬆️
internal/core/src/segcore/MemoryPlannerTest.cpp	99.50% <96.15%> (-0.50%)	⬇️
...gcore/storagev2translator/GroupChunkTranslator.cpp	87.61% <0.00%> (-2.39%)	⬇️
...re/storagev2translator/ManifestGroupTranslator.cpp	77.31% <0.00%> (-2.47%)	⬇️

... and 12 files with indirect coverage changes

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