feat(storage): implement atomic compose via DeleteSourceObjects

[PranjalC100]

Description

This pull request implements an atomic compose operation by introducing a DeleteSourceObjects boolean inside the ComposeObjectsRequest. It leverages the new GCS Compose API functionality that allows atomically deleting source objects on the server-side the exact millisecond a compose operation completes.

This change fundamentally resolves race conditions, early deletion billing spikes, and "soft delete" charges commonly encountered during append-heavy AI/ML workloads.

Key Changes:

• Dependency Upgrade: Upgraded cloud.google.com/go/storage to v1.63.0 to gain access to the DeleteSourceObjects attribute on the Compose request.
• Core Implementation: Passed the DeleteSourceObjects: true flag during GCS object composition via ComposeObjectCreator and removed the manual DeleteObject deferred calls.
• Garbage Collector (Fallback): The background garbage collector was intentionally retained. If a ComposeObjects API call fails mid-flight, the server-side deletion never triggers. The unmodified garbage collector acts as an excellent, zero-overhead resilient fallback to find and delete these rare orphaned chunks.
• Mock Updates: Updated the ComposeObjects signature in fake/bucket.go to explicitly simulate atomic slice deletion of source objects inside integration test mocks.

Link to the issue in case of a bug fix:

b/430606937

Testing details

1. Manual - Executed standard test sweeps and compilation checks locally via make build.
2. Unit tests - Confirmed compose_object_creator_test.go properly asserts for req.DeleteSourceObjects and that legacy DeleteObject expectations were removed. Added a new explicit DeleteSourceObjects unit test suite to internal/storage/fake/testing/bucket_tests.go. All tests passed successfully.
3. Integration tests - NA

Any backward incompatible change? If so, please explain.

N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.68%. Comparing base (b609bb2) to head (482c0ab).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4804       +/-   ##
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+ Coverage        0   83.68%   +83.68%     
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  Files           0      168      +168     
  Lines           0    20897    +20897     
===========================================
+ Hits            0    17487    +17487     
- Misses          0     2758     +2758     
- Partials        0      652      +652     

Flag	Coverage Δ
unittests	83.68% <100.00%> (?)

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