Port over BashSession and tests (#7)

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

Author: xingyaoww

.openhands/microagents/repo.md

@@ -136,6 +136,7 @@ This project contains my tasks of completely refactor [OpenHands](https://github
     - read [openhands/core/runtime/tools/str_replace_editor/impl.py](openhands/core/runtime/tools/str_replace_editor/impl.py) for tool execute_fn
     - read [openhands/core/runtime/tools/str_replace_editor/definition.py](openhands/core/runtime/tools/str_replace_editor/definition.py) for how do we define a tool
     - read [openhands/core/runtime/tools/str_replace_editor/__init__.py](openhands/core/runtime/tools/str_replace_editor/__init__.py) for how we define each tool module
+- Check how we perform tests in .github/workflows/tests.yml
 - ...
 </TASK>
 

openhands/core/runtime/schema.py

@@ -132,3 +132,8 @@ class ObservationBase(Schema):
     """Base schema for output observation."""
 
     model_config = ConfigDict(extra="allow")
+
+    @property
+    def agent_observation(self) -> str:
+        """Get the observation string to show to the agent."""
+        raise NotImplementedError("Subclasses must implement agent_observation")