Move test_marketplace.py to tests/sdk/marketplace/

Restructure tests to match the new module structure:
- Move tests/sdk/plugin/test_marketplace.py -> tests/sdk/marketplace/test_marketplace.py
- Update imports to use openhands.sdk.marketplace (canonical location)

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent

tests/sdk/plugin/test_marketplace.py tests/sdk/marketplace/test_marketplace.pytests/sdk/plugin/test_marketplace.py renamed to tests/sdk/marketplace/test_marketplace.py

@@ -4,14 +4,14 @@
 
 import pytest
 
-from openhands.sdk.plugin import (
+from openhands.sdk.marketplace import (
     Marketplace,
     MarketplaceMetadata,
     MarketplaceOwner,
     MarketplacePluginEntry,
     MarketplacePluginSource,
-    PluginAuthor,
 )
+from openhands.sdk.plugin import PluginAuthor
 
 
 class TestMarketplaceOwner: