fix(error-handling): handle FileNotFoundError in robot.helper_actor_config

Wrap ActorConfiguration.from_file() in a try...except FileNotFoundError
block in robot/helper_actor_config.py. When the config file path does not
exist, print a user-friendly error message to sys.stderr and exit with
code 1 instead of crashing with an unhandled exception and raw traceback.

Also adds a Robot Framework test case 'Missing Config File Exits With
Non-Zero Code And Stderr Message' to robot/actor_configuration.robot to
cover the new error path.

ISSUES CLOSED: #2829

robot/actor_configuration.robot

@@ -40,3 +40,9 @@ V2 Actor Config Produces Provider And Graph Descriptor
     List Should Contain Value    ${payload['graph_keys']}    agents
     List Should Contain Value    ${payload['graph_keys']}    routes
     Should Be Equal As Numbers    ${payload['options']['temperature']}    0.5
+
+Missing Config File Exits With Non-Zero Code And Stderr Message
+    ${missing}=    Set Variable    ${OUTPUT DIR}/does_not_exist_actor.yaml
+    ${result}=    Run Process    ${PYTHON}    robot/helper_actor_config.py    ${missing}
+    Should Not Be Equal As Integers    ${result.rc}    0
+    Should Contain    ${result.stderr}    Error: Config file not found at '${missing}'

robot/helper_actor_config.py

@@ -12,7 +12,11 @@ def main() -> None:
         raise SystemExit("config path required")
 
     config_path = Path(sys.argv[1])
-    config = ActorConfiguration.from_file(path=config_path)
+    try:
+        config = ActorConfiguration.from_file(path=config_path)
+    except FileNotFoundError:
+        print(f"Error: Config file not found at '{config_path}'", file=sys.stderr)
+        sys.exit(1)
     payload = {
         "provider": config.provider,
         "model": config.model,