Make ResourceSessionFactory generic over its session type (#1007)

ResourceSessionFactory.create() was typed as returning the base ResourceSession,
so callers lost the concrete session type and had to cast. Parameterize the
factory over a SessionT bound to ResourceSession, so a subclass like
OpenCodeSessionFactory(ResourceSessionFactory[OpenCodeSession]) types create()
as returning OpenCodeSession. Subclassing without a type argument stays valid
and behaves as before, so existing envs are unaffected.

Author: sergiopaniego

envs/opencode_env/harness.py

@@ -189,7 +189,7 @@ def fetch_proxy_trace(self) -> list[dict[str, Any]]:
         return records
 
 
-class OpenCodeSessionFactory(ResourceSessionFactory):
+class OpenCodeSessionFactory(ResourceSessionFactory[OpenCodeSession]):
     """Produce isolated per-rollout :class:`OpenCodeSession` instances.
 
     The factory owns sandbox provisioning, opencode install, config injection,

src/openenv/core/harness/__init__.py

@@ -12,7 +12,7 @@
 import math
 from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
-from typing import Any, Callable, Protocol
+from typing import Any, Callable, Generic, Protocol, TypeVar
 
 from ..client_types import StepResult
 from ..env_server.mcp_types import JsonRpcErrorCode, JsonRpcResponse, Tool
@@ -141,16 +141,25 @@ def close(self) -> None:
         """Release session resources."""
 
 
-class ResourceSessionFactory(ABC):
-    """Factory for producing isolated per-rollout sessions."""
+SessionT = TypeVar("SessionT", bound=ResourceSession)
+
+
+class ResourceSessionFactory(ABC, Generic[SessionT]):
+    """Factory for producing isolated per-rollout sessions.
+
+    Generic over the concrete session type it creates, so a subclass such as
+    ``OpenCodeSessionFactory(ResourceSessionFactory[OpenCodeSession])`` types
+    ``create`` as returning ``OpenCodeSession``. Subclassing without a type
+    argument stays valid and behaves as before.
+    """
 
     @abstractmethod
     def create(
         self,
         task: Any,
         seed: int | None = None,
         episode_id: str | None = None,
-    ) -> ResourceSession:
+    ) -> SessionT:
         """Create one isolated resource session for a rollout."""