[FEATURE] Add endpoint_url parameter to S3SessionManager for S3-compatible storage backends

[tealgreen0503]

Problem Statement

S3SessionManager currently creates its internal boto3 S3 client with limited configuration options (boto_session, boto_client_config, region_name), but does not accept endpoint_url. This makes it impossible to use S3SessionManager with S3-compatible storage backends such as MinIO or LocalStack, which require a custom endpoint URL.

This is a very common need for local development and testing. For comparison, BedrockModel already supports endpoint_url as a first-class constructor parameter (bedrock.py L131), so there is precedent for this pattern within the SDK.

Proposed Solution

Add an explicit endpoint_url parameter to S3SessionManager.__init__() and forward it to the internal session.client() call, matching the existing BedrockModel pattern.

session_manager = S3SessionManager(
    session_id="my-session",
    bucket="my-bucket",
    endpoint_url="http://localhost:9000",  # New parameter
)

This approach:

• Is consistent with the existing BedrockModel constructor pattern
• Is a minimal, backwards-compatible change

Use Case

1. Local development with MinIO: Developers commonly run MinIO as a local S3-compatible object store for development. They need to set endpoint_url to http://localhost:9000 instead of AWS S3.

   session_manager = S3SessionManager(
       session_id="dev-session",
       bucket="dev-bucket",
       endpoint_url="http://localhost:9000",
   )
   agent = Agent(session_manager=session_manager)

2. VPC endpoints / PrivateLink: In production AWS environments, teams may need to route S3 traffic through VPC endpoints using custom endpoint URLs — the same use case that BedrockModel already supports.

Alternatives Solutions

No response

Additional Context

Current S3SessionManager client creation (s3_session_manager.py L85):

self.client = session.client(service_name="s3", config=client_config)

BedrockModel already forwards endpoint_url (bedrock.py L171-176):

self.client = session.client(
    service_name="bedrock-runtime",
    config=client_config,
    endpoint_url=endpoint_url,
    region_name=resolved_region,
)

The proposed change would simply align S3SessionManager with this established pattern.