feat: add concurrent S3 message reading to list_messages

[Unshure]

Motivation

Session managers that store messages in S3 can experience slow performance when listing many messages, as each message is read sequentially from S3. For sessions with dozens or hundreds of messages, this sequential reading creates a significant performance bottleneck that impacts user experience and system responsiveness.

Changes

This PR modifies the S3SessionManager.list_messages() method to read S3 objects concurrently, providing substantial performance improvements for sessions with multiple messages.

The implementation uses the existing run_async() utility to spawn a threaded event loop, then leverages asyncio.to_thread() and asyncio.gather() to perform concurrent S3 reads. The external API remains completely synchronous, requiring no changes to consuming code.

Public API Changes

None. The external API signature and behavior of list_messages() remain unchanged:

def list_messages(
    self, session_id: str, agent_id: str, limit: Optional[int] = None, offset: int = 0, **kwargs: Any
) -> List[SessionMessage]:
    """List messages for an agent with pagination from S3.
    
    Uses concurrent async reading for improved performance when reading multiple messages.
    """

Breaking Changes

None. This is a backward-compatible performance improvement.

Implementation Notes

Initially explored using aiobotocore for native async S3 operations, but discovered that the moto library used for testing doesn't fully support aiobotocore's async client. Switched to asyncio.to_thread() approach which:

• Provides the same concurrent execution benefits
• Maintains full compatibility with existing test infrastructure
• Requires no additional dependencies
• Works seamlessly with the existing boto3 client

Resolves #1416

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/session/s3_session_manager.py	83.33%	2 Missing and 3 partials ⚠️

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