fix(memory): guard retrieve_customer_context against empty message content

[dtaniwaki]

Thanks for maintaining this project! Would appreciate a review when you get a chance.

Fixes #543

Problem

AgentCoreMemorySessionManager.retrieve_customer_context (registered as a MessageAddedEvent callback) indexes the first content block of the last message without checking that the content list is non-empty:

if not messages or messages[-1].get("role") != "user" or "text" not in messages[-1].get("content")[0]:
    return None

When the last message has an empty content list ([]), messages[-1].get("content")[0] raises IndexError. Because this runs inside a MessageAddedEvent hook, the exception propagates out of the event loop and aborts the whole agent invocation (surfacing as a 500 on AgentCore Runtime). The existing guard handles not messages but not an empty content list. (See #543 for a runnable end-to-end reproduction.)

Solution

Check that content is non-empty before indexing the first block, returning early otherwise. Added a regression test (test_retrieve_customer_context_empty_content) that fails without the guard and passes with it. All existing tests in the session manager suite still pass, and ruff check / ruff format are clean.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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  Hits            ?     7666           
  Misses          ?      585           
  Partials        ?      324           

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[nborges-aws]

Breaking change bot failure is false alarm. Reported "No Breaking Changes Detected", but errored while attempting to post comment to PR, causing workflow to fail.

PR looks good to me. Thanks for the fix!