fix: normalize snake_case/camelCase in passthrough methods for consistent SDK API

[Hweinstock]

*Issue #, if available:*#336

Description of changes:

Problem

MemoryClient and MemorySessionManager expose some methods via __getattr__ passthrough to boto3, which requires camelCase kwargs (memoryId, clientToken), while explicit SDK methods accept snake_case (memory_id,
client_token). Users have no way to know which convention to use without reading the source.

See attached issue for an example.

Solution

• Leverage a middleware layer to accept both conventions.
• camelCase still works (backward compat), snake_case is converted to camelCase before forwarding to boto3.
• Passing both forms for the same param (e.g. memory_id and memoryId) raises TypeError. Malformed snake_case (e.g. a__b, _private) is rejected with ValueError.
• provide minimal validation of snake_case input to avoid accepting bad input.

Follows the existing __getattr__ pattern — no new public API surface.

Testing

• unit tests
• 1 integ test updated to use snake_case for update_memory passthrough (e2e verification)

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@f8710fa). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/bedrock_agentcore/memory/client.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #348   +/-   ##
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  Coverage        ?   90.96%           
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  Files           ?       44           
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  Branches        ?      628           
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  Hits            ?     3724           
  Misses          ?      203           
  Partials        ?      167           

Flag	Coverage Δ
unittests	90.96% <96.55%> (?)

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