S3Backend: tag boto3 user-agent and honor end_point_url without credentials

[goanpeca]

Summary

Three changes to S3Backend in internvl_chat_gpt_oss/internvl/utils/s3_fileio.py:

1. Add user_agent_extra="internvl-chat-gpt-oss" to the botocore.Config so InternVL TCS traffic is identifiable in server-side logs of any S3 provider (AWS, Backblaze B2, Ceph RGW, Cloudflare R2, CoreWeave, MinIO, etc.).

2. Fix end_point_url being silently dropped in the no-credentials path. The original else-branch built boto3.client('s3') with no endpoint and no Config, so a petreloss.conf cluster with endpoint_url but no access_key (relying on boto3's default credential chain such as env vars or an IAM role) was silently routed to AWS S3 instead of the configured cluster.

3. Collapse the if/else into a single boto3.client(...) call. With both branches now passing the same endpoint_url and Config, the only difference was the credential args, which boto3 treats as no-ops when None.

Also a one-line docstring update listing common S3-compatible providers, and the bachend → backend typo fix.

Test plan

No test directory exists in the repo, so verified manually for the four call shapes (no / with end_point_url, no / with credentials). In every case the boto3 client carries user_agent_extra="internvl-chat-gpt-oss", read_timeout=5, connect_timeout=2, and the correct endpoint URL.

No new runtime dependencies.