fix: handle JSON parsing errors in planner agent

Add try-except for json.JSONDecodeError in parse_agent_tasks() to
gracefully handle malformed JSON from LLM responses (e.g. Gemini 2.5
Flash via OpenRouter). Returns empty list on parse failure, which
triggers the existing retry mechanism in make_plan().

Add test suite for parse_agent_tasks() covering valid JSON, malformed
JSON, truncated JSON, and invalid agent name cases.

sources/agents/planner_agent.py

@@ -76,7 +76,12 @@ class PlannerAgent(Agent):
         if blocks == None:
             return []
         for block in blocks:
-            line_json = json.loads(block)
+            try:
+                line_json = json.loads(block)
+            except json.JSONDecodeError as e:
+                self.logger.warning(f"Failed to parse JSON block: {e}")
+                pretty_print(f"JSON parsing error: {e}", color="warning")
+                return []
             if 'plan' in line_json:
                 for task in line_json['plan']:
                     if task['agent'].lower() not in [ag_name.lower() for ag_name in self.agents.keys()]: