fix: prevent MinIO health check failure in multi-bucket mode (#12446)

### What problem does this PR solve?

- Fixes the health check failure in multi-bucket MinIO environments.
Previously, health checks would fail because the default
"ragflow-bucket" did not exist. This caused false negatives for system
health.

- Also removes the _health_check write in single-bucket mode to avoid
side effects (minor optimization).

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/utils/minio_conn.py

@@ -18,7 +18,7 @@ import logging
 import time
 from minio import Minio
 from minio.commonconfig import CopySource
-from minio.error import S3Error
+from minio.error import S3Error, ServerError, InvalidResponseError
 from io import BytesIO
 from common.decorator import singleton
 from common import settings
@@ -97,19 +97,28 @@ class RAGFlowMinio:
         self.conn = None
 
     def health(self):
-        bucket = self.bucket if self.bucket else "ragflow-bucket"
-        fnm = "_health_check"
-        if self.prefix_path:
-            fnm = f"{self.prefix_path}/{fnm}"
-        binary = b"_t@@@1"
-        # Don't try to create bucket - it should already exist
-        # if not self.conn.bucket_exists(bucket):
-        #     self.conn.make_bucket(bucket)
-        r = self.conn.put_object(bucket, fnm,
-                                 BytesIO(binary),
-                                 len(binary)
-                                 )
-        return r
+        """
+        Check MinIO service availability.
+        """
+        try:
+            if self.bucket:
+                # Single-bucket mode: check bucket exists only (no side effects)
+                exists = self.conn.bucket_exists(self.bucket)
+
+                # Historical:
+                # - Previously wrote "_health_check" to verify write permissions
+                # - Previously auto-created bucket if missing
+
+                return exists
+            else:
+                # Multi-bucket mode: verify MinIO service connectivity
+                self.conn.list_buckets()
+                return True
+        except (S3Error, ServerError, InvalidResponseError):
+            return False
+        except Exception as e:
+            logging.warning(f"Unexpected error in MinIO health check: {e}")
+            return False
 
     @use_default_bucket
     @use_prefix_path