feat(session): add endpoint_url parameter to S3SessionManager

[Zelys-DFKH]

Description

`S3SessionManager` creates its boto3 S3 client without accepting an `endpoint_url` parameter. This makes it impossible to use the session manager with S3-compatible storage backends such as MinIO or LocalStack, which are common in local development and VPC endpoint configurations.

`BedrockModel` already accepts `endpoint_url` as a first-class constructor parameter and forwards it to its boto3 client. This change applies the same pattern to `S3SessionManager`.

Changes:

• Add `endpoint_url: str | None = None` to `S3SessionManager.init()`
• Forward it to `session.client(service_name="s3", ...)`

# Local development with MinIO
session_manager = S3SessionManager(
    session_id="dev-session",
    bucket="dev-bucket",
    endpoint_url="http://localhost:9000",
)

# VPC endpoint / PrivateLink
session_manager = S3SessionManager(
    session_id="prod-session",
    bucket="prod-bucket",
    endpoint_url="https://bucket.vpce-abc123.s3.us-east-1.vpce.amazonaws.com",
)

When `endpoint_url` is not provided (the default), behavior is unchanged.

Related Issues

Fixes #1794

Documentation PR

N/A

Type of Change

New feature

Testing

• Added `test_init_s3_session_manager_with_endpoint_url` verifying that `endpoint_url` is forwarded to the boto3 client creation call
• All 47 existing S3SessionManager tests pass

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

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