fix: add object path validation util

This adds an object name validation util to check if the object
path would resolve to a path outside of the bucket directory.

S3 returns Bad Request for these type of paths:
 % aws s3api put-object --bucket mybucket --key test/../../hello
An error occurred (400) when calling the PutObject operation: Bad Request

s3api/utils/path_test.go

@@ -0,0 +1,64 @@
+// Copyright 2025 Versity Software
+// This file is licensed under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package utils_test
+
+import (
+	"testing"
+
+	"github.com/versity/versitygw/s3api/utils"
+)
+
+func TestIsObjectNameValid(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  bool
+	}{
+		// valid names
+		{"simple file", "file.txt", true},
+		{"nested file", "dir/file.txt", true},
+		{"absolute nested file", "/dir/file.txt", true},
+		{"trailing slash", "dir/", true},
+		{"slash prefix", "/file.txt", true}, // treated as local after joined with bucket
+		{"dot slash prefix", "./file.txt", true},
+
+		// invalid names
+		{"dot dot only", "..", false},
+		{"dot only", ".", false},
+		{"dot slash", "./", false},
+		{"dot slash dot dot", "./..", false},
+		{"cleans to dot", "./../.", false},
+		{"empty", "", false},
+		{"file escapes 1", "../file.txt", false},
+		{"file escapes 2", "dir/../../file.txt", false},
+		{"file escapes 3", "../../../file.txt", false},
+		{"dir escapes 1", "../dir/", false},
+		{"dir escapes 2", "dir/../../dir/", false},
+		{"dir escapes 3", "../../../dir/", false},
+		{"dot escapes 1", "../.", false},
+		{"dot escapes 2", "dir/../../.", false},
+		{"dot escapes 3", "../../../.", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := utils.IsObjectNameValid(tt.input)
+			if got != tt.want {
+				t.Errorf("%v: IsObjectNameValid(%q) = %v, want %v",
+					tt.name, tt.input, got, tt.want)
+			}
+		})
+	}
+}