feat: parallelize retrieve memories API calls for multiple namespaces to improve latency

[colifran]

Description of changes:

• Refactored retrieve_customer_context to retrieve memories from multiple namespaces concurrently using ThreadPoolExecutor instead of sequentially
• Extracted namespace retrieval logic into a helper function retrieve_for_namespace for cleaner concurrent execution
• Added per-namespace error handling so failures in one namespace don't block retrieval from others (graceful degradation)
• Removed unneeded braces around len(all_context)

This improves response time for support queries when multiple namespaces are configured, reducing total retrieval time from the sum of all calls to the maximum of any single call.

Testing:

• I performed manual testing with tracing enabled this screenshot shows the current implementation is making retrieve_memories API calls sequentially:

• This screenshot shows that retrieve_memories API calls are made in parallel with the updated code:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[jona62]

LGTM!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@988ca8f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ore/memory/integrations/strands/session_manager.py	86.95%	0 Missing and 3 partials ⚠️

Additional details and impacted files

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  Coverage        ?   90.32%           
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  Files           ?       25           
  Lines           ?     2718           
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  Hits            ?     2455           
  Misses          ?      153           
  Partials        ?      110           

Flag	Coverage Δ
unittests	90.32% <86.95%> (?)

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