fix(memory): fix last_k_turns

[akshseh]

Issue #, if available: awslabs/amazon-bedrock-agentcore-samples#172

Description of changes:

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[jona62]

LGTM!!!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@9653a1f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/bedrock_agentcore/memory/client.py	72.72%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #62   +/-   ##
=======================================
  Coverage        ?   88.21%           
=======================================
  Files           ?       16           
  Lines           ?     1544           
  Branches        ?      231           
=======================================
  Hits            ?     1362           
  Misses          ?       89           
  Partials        ?       93           

Flag	Coverage Δ
unittests	88.21% <72.72%> (?)

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