fix: shared default plugin manager and cost manager instances among multiple invocations

PiperOrigin-RevId: 791303349

src/google/adk/agents/invocation_context.py

@@ -20,6 +20,8 @@ import uuid
 from google.genai import types
 from pydantic import BaseModel
 from pydantic import ConfigDict
+from pydantic import Field
+from pydantic import PrivateAttr
 
 from ..artifacts.base_artifact_service import BaseArtifactService
 from ..auth.credential_service.base_credential_service import BaseCredentialService
@@ -154,10 +156,12 @@ class InvocationContext(BaseModel):
   run_config: Optional[RunConfig] = None
   """Configurations for live agents under this invocation."""
 
-  plugin_manager: PluginManager = PluginManager()
+  plugin_manager: PluginManager = Field(default_factory=PluginManager)
   """The manager for keeping track of plugins in this invocation."""
 
-  _invocation_cost_manager: _InvocationCostManager = _InvocationCostManager()
+  _invocation_cost_manager: _InvocationCostManager = PrivateAttr(
+      default_factory=_InvocationCostManager
+  )
   """A container to keep track of different kinds of costs incurred as a part
   of this invocation.
   """