Use timezone-aware datetime for dataset lifecycle comparisons

[Copilot]

Addresses feedback from #1211: datetime comparisons in dataset lifecycle operations were mixing naive and timezone-aware datetime objects, causing inconsistent behavior.

Changes

• dataset_lifecycle_ops.py: Changed datetime.now() to datetime.now(timezone.utc) for timezone-aware comparisons
• test_dataset_lifecycle_ops.py: Updated test fixtures to use timezone-aware datetimes for consistency

Context

The last_updated field is set using datetime.now(tz=timezone.utc) in dataset_manager.py. Comparing it with a naive datetime from datetime.now() can fail or produce incorrect results depending on the Python version and database driver.

# Before
now = datetime.now()  # naive datetime
if _.last_updated and (now - _.last_updated) > delta:  # comparing naive with tz-aware

# After  
now = datetime.now(timezone.utc)  # timezone-aware datetime
if _.last_updated and (now - _.last_updated) > delta:  # consistent comparison

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.10%. Comparing base (4d609d9) to head (adf1aec).
⚠️ Report is 1 commits behind head on dataset-cache-cleanup.

Additional details and impacted files

@@                    Coverage Diff                    @@
##           dataset-cache-cleanup    #1214      +/-   ##
=========================================================
+ Coverage                  86.04%   86.10%   +0.06%     
=========================================================
  Files                         95       95              
  Lines                       3261     3261              
  Branches                     373      373              
=========================================================
+ Hits                        2806     2808       +2     
+ Misses                       380      378       -2     
  Partials                      75       75              

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