fix: S3 listing NextMarker missing intermediate directory component (#8089)

* fix: S3 listing NextMarker missing intermediate directory component

When listing with nested prefixes like "character/member/", the NextMarker
was incorrectly constructed as "character/res024/" instead of
"character/member/res024/", causing continuation requests to fail.

Root cause: The code at line 331 was constructing NextMarker as:
  nextMarker = requestDir + "/" + nextMarker

This worked when nextMarker already contained the full relative path,
but failed when it was just the entry name from the innermost recursion.

Fix: Include the prefix component when constructing NextMarker:
  if prefix != "" {
      nextMarker = requestDir + "/" + prefix + "/" + nextMarker
  }

This ensures the full path is always constructed correctly for both:
- CommonPrefix entries (directories)
- Regular entries (files)

Also includes fix for cursor.prefixEndsOnDelimiter state leak that was
causing sibling directories to be incorrectly listed.

* test: add regression tests for NextMarker construction

Add comprehensive unit tests to verify NextMarker is correctly constructed
with nested prefixes. Tests cover:
- Regular entries with nested prefix (character/member/res024)
- CommonPrefix entries (directories)
- Edge cases (no requestDir, no prefix, deeply nested)

These tests ensure the fix prevents regression of the bug where
NextMarker was missing intermediate directory components.

weed/s3api/s3api_object_handlers_list.go

@@ -322,13 +322,21 @@ func (s3a *S3ApiServer) listFilerEntries(bucket string, originalPrefix string, m
 			if cursor.isTruncated && lastEntryWasCommonPrefix && lastCommonPrefixName != "" {
 				// For CommonPrefixes, NextMarker should include the trailing slash
 				if requestDir != "" {
-					nextMarker = requestDir + "/" + lastCommonPrefixName + "/"
+					if prefix != "" {
+						nextMarker = requestDir + "/" + prefix + "/" + lastCommonPrefixName + "/"
+					} else {
+						nextMarker = requestDir + "/" + lastCommonPrefixName + "/"
+					}
 				} else {
 					nextMarker = lastCommonPrefixName + "/"
 				}
 			} else if cursor.isTruncated {
 				if requestDir != "" {
-					nextMarker = requestDir + "/" + nextMarker
+					if prefix != "" {
+						nextMarker = requestDir + "/" + prefix + "/" + nextMarker
+					} else {
+						nextMarker = requestDir + "/" + nextMarker
+					}
 				}
 			}