GCS List method doesn't add slash suffix to prefix

[hirokawai]

Same issue as #15 (Huawei OBS), but for Google Cloud Storage.

Problem

GoogleCloudStorage.List() in oss/gcsblob/gcs.go does not append a trailing slash to the prefix before listing
objects. This causes prefix collisions when multiple directories share a common prefix.

In dify-plugin-daemon, plugin/ and plugin_packages/ both exist in the same bucket. When List("plugin") is
called, GCS returns objects from both prefixes. After TrimPrefix, objects from plugin_packages/ become
_packages/... which produces invalid plugin identifiers.

ERROR dify-plugin-daemon installed_bucket.go:81 failed to create PluginUniqueIdentifier from path
path=_packages/lifull/my_plugin:0.0.1@e5bc5c2c...
error="plugin_unique_identifier is not valid"

Fix

Add the same slash suffix guard that the S3 implementation already has:

func (g *GoogleCloudStorage) List(prefix string) ([]oss.OSSPath, error) {
    if !strings.HasSuffix(prefix, "/") {
        prefix = prefix + "/"
    }
    // ... rest unchanged