feat(marketplace): expose load_plugin on base conversation

Add a BaseConversation load_plugin API surface with a default unsupported implementation for custom conversation subclasses.

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

Author: openhands-agent

openhands-sdk/openhands/sdk/conversation/base.py

@@ -378,6 +378,19 @@ def execute_tool(self, tool_name: str, action: Action) -> Observation:
         """
         ...
 
+    def load_plugin(self, plugin_ref: str) -> None:
+        """Load a plugin from a registered marketplace.
+
+        Implementations that support marketplace-registered plugins resolve the
+        reference against the conversation agent's registered marketplaces and
+        merge the plugin's skills, hooks, and MCP configuration into the agent.
+
+        Args:
+            plugin_ref: Plugin reference, either ``plugin-name`` or
+                ``plugin-name@marketplace-name``.
+        """
+        raise NotImplementedError("This conversation does not support loading plugins")
+
     @abstractmethod
     def fork(
         self,

tests/sdk/conversation/test_base_span_management.py

@@ -5,6 +5,8 @@
 from unittest.mock import MagicMock, patch
 from uuid import UUID
 
+import pytest
+
 from openhands.sdk.conversation.base import BaseConversation
 from openhands.sdk.conversation.conversation_stats import ConversationStats
 from openhands.sdk.conversation.types import TraceMetadataValue
@@ -74,6 +76,13 @@ def fork(self, **kwargs: Any) -> "MockConversation":
         raise NotImplementedError("Mock fork not implemented")
 
 
+def test_base_conversation_load_plugin_default_not_supported():
+    conversation = MockConversation()
+
+    with pytest.raises(NotImplementedError, match="does not support loading plugins"):
+        conversation.load_plugin("plugin@marketplace")
+
+
 def test_base_conversation_span_management():
     """Test that BaseConversation properly manages span state to prevent double-ending."""  # noqa: E501