fix(dynamodb): add test for put_item with ReturnValues='ALL_OLD' on new items

[thakoreh]

Summary

Related to #9757

Adds a test case verifying that put_item() with ReturnValues='ALL_OLD' returns an empty Attributes map when creating a new item (not replacing an existing one).

Behavior

Per AWS DynamoDB documentation:

• ReturnValues='ALL_OLD' returns the old item attributes if an item was replaced
• For new items (no existing item to replace), returns {"Attributes": {}}

Test Added

test_put_item_return_values_all_old_new_item - verifies the empty Attributes response for new items.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.11%. Comparing base (c233a61) to head (fc958ce).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9778   +/-   ##
=======================================
  Coverage   93.11%   93.11%           
=======================================
  Files        1308     1308           
  Lines      118798   118798           
=======================================
+ Hits       110615   110617    +2     
+ Misses       8183     8181    -2     

Flag	Coverage Δ
servertests	28.89% <ø> (ø)
unittests	93.08% <ø> (+<0.01%)	⬆️

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[bpandola]

Doesn't this comment contradict #9757, which says "in the case of a new entry, DynamoDB itself doesn't include an Attributes attribute in the response"? You are saying here that the key should not be omitted--were you able to verify that against a real AWS backend?