feat(session): add put_object_kwargs support to S3SessionManager

[gauravSsinha]

Description

Adds support for passing custom parameters to S3 put_object calls in S3SessionManager. This enables configuring server-side encryption (SSE-KMS), storage class, tagging, and other S3 PutObject parameters when storing session data.

Changes

• Added put_object_kwargs extraction from **kwargs in S3SessionManager.__init__()
• Updated _write_s3_object() to spread put_object_kwargs into all put_object calls
• Added 4 unit tests covering initialization and S3 write behavior

Usage

from strands.session import S3SessionManager

session_manager = S3SessionManager(
    session_id="my-session",
    bucket="my-bucket",
    put_object_kwargs={
        "ServerSideEncryption": "aws:kms",
        "SSEKMSKeyId": "arn:aws:kms:us-east-1:123456789012:key/12345678-1234-1234-1234-123456789012"
    }
)

Design Decision

Following the maintainer's guidance in #1247, put_object_kwargs is passed via **kwargs and stored as an instance variable. This approach:

• Is backwards-compatible (no signature change)
• Follows the existing extensibility pattern (**kwargs already accepted)
• Scales with the S3 PutObject API without needing individual parameters

Testing

• All 50 existing S3SessionManager tests pass
• 4 new tests added covering:
  • Initialization with put_object_kwargs
  • Default empty dict when not provided
  • Successful S3 writes with encryption params
  • Successful S3 writes with StorageClass param

Closes #1247