(enh) Improve CodeActAgent's file editing reliability (#3610)

* improve file editing prompts and unit test
converted most raise calls to a _output_error call in file_ops.py

* tweaks in test_agent_skill.py wrt to SEP separator

* tweaked the separator

* remove server runtime remnants and TEST_RUNTIME references

* restore use of TEST_RUNTIME args and variables

* fix integration tests

* added hint to properly escape docstrings

* revert latest prompt change

---------

Co-authored-by: Xingyao Wang <xingyao6@illinois.edu>

openhands/server/session/agent.py

@@ -47,7 +47,7 @@ class AgentSession:
             start_event: The start event data (optional).
         """
         if self.controller or self.runtime:
-            raise Exception(
+            raise RuntimeError(
                 'Session already started. You need to close this session and start a new one.'
             )
         await self._create_security_analyzer(config.security.security_analyzer)
@@ -87,7 +87,7 @@ class AgentSession:
         if self.runtime is not None:
             raise Exception('Runtime already created')
 
-        logger.info(f'Using runtime: {runtime_name}')
+        logger.info(f'Initializing runtime `{runtime_name}` now...')
         runtime_cls = get_runtime_cls(runtime_name)
         self.runtime = runtime_cls(
             config=config,
@@ -107,9 +107,11 @@ class AgentSession:
     ):
         """Creates an AgentController instance."""
         if self.controller is not None:
-            raise Exception('Controller already created')
+            raise RuntimeError('Controller already created')
         if self.runtime is None:
-            raise Exception('Runtime must be initialized before the agent controller')
+            raise RuntimeError(
+                'Runtime must be initialized before the agent controller'
+            )
 
         logger.info(f'Agents: {agent_configs}')
         logger.info(f'Creating agent {agent.name} using LLM {agent.llm.config.model}')
@@ -135,3 +137,4 @@ class AgentSession:
             logger.info(f'Restored agent state from session, sid: {self.sid}')
         except Exception as e:
             logger.info(f'Error restoring state: {e}')
+        logger.info('Agent controller initialized.')