feat: add region parameter support to MinIO connection (#13954)

## Summary
- Add optional `region` parameter to `Minio()` client constructor in
`rag/utils/minio_conn.py`
- Reads from `MINIO.region` in settings, defaults to `None` when not
configured
- Required by some S3-compatible storage services (e.g., AWS S3, Tencent
COS) for proper bucket access

## Motivation
When using RAGFlow with S3-compatible storage that requires a region
(such as AWS S3 or Tencent Cloud COS), the MinIO client fails to access
buckets because the `region` parameter is not passed through.

The `Minio()` Python client already supports the `region` parameter
natively — this PR simply wires it up from the RAGFlow configuration.

## Changes
- `rag/utils/minio_conn.py`: Pass `region=settings.MINIO.get("region",
None) or None` to `Minio()` constructor

## Backward Compatibility
- No breaking changes. When `region` is not configured, it defaults to
`None`, preserving the existing behavior exactly.

## Test Plan
- [ ] Verified with MinIO (no region set) — works as before
- [x] Verified with S3-compatible storage requiring region — bucket
access succeeds

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Enhanced MinIO client initialization with regional configuration
support for improved compatibility with region-specific deployments.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Jarry Wang <code-better-life@users.noreply.github.com>
Co-authored-by: Jin Hai <haijin.chn@gmail.com>

rag/utils/minio_conn.py

@@ -106,6 +106,7 @@ class RAGFlowMinio:
                 access_key=settings.MINIO["user"],
                 secret_key=settings.MINIO["password"],
                 secure=secure,
+                region=settings.MINIO.get("region", None) or None,
                 http_client=http_client,
             )
         except Exception: