fix(service): Filter expired objects in the S3-compatible backend

[jan-auer]

The GCS and BigTable backends filter out expired objects at read time — returning None when the stored expiry timestamp is in the past. The S3-compatible backend was missing this check, so TTL and TTI objects remained readable past their deadline until the storage lifecycle rule physically deleted them (up to ~48h later).

This lifts the expire_at parsing out of the TTI-only block and adds the same expiry guard before the TTI bump, matching the GCS pattern. The response body is drained before returning None to avoid connection leaks.

Ref FS-403

[linear-code]

FS-403

[codecov]

Codecov Report

❌ Patch coverage is 0% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.46%. Comparing base (e88b7f8) to head (56d980d).

Files with missing lines	Patch %	Lines
objectstore-service/src/backend/s3_compatible.rs	0.00%	45 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #569      +/-   ##
==========================================
- Coverage   87.67%   87.46%   -0.22%     
==========================================
  Files          93       93              
  Lines       15113    15152      +39     
==========================================
+ Hits        13251    13252       +1     
- Misses       1862     1900      +38     

Components	Coverage Δ
Rust Backend	92.14% <0.00%> (-0.31%)	⬇️
Rust Client	79.89% <ø> (ø)
Python Client	90.20% <ø> (ø)

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.