fix: prevent SessionStateEncoder crash from non-serializable datetime on args

Root cause: bot_flow.py injected configs.args.global_start_time = datetime.now()
which polluted the args namespace. SessionStateEncoder blindly serialized
args.__dict__ via json.dump, which crashed mid-write on the datetime object,
leaving sessions.json truncated/corrupt. Every subsequent restart failed.

Fixes:
- Remove global_start_time from configs.args (only lives on engine class vars)
- Harden SessionStateEncoder with _sanitize_value() to convert any
  non-JSON-serializable type (datetime, timedelta, arbitrary objects) to strings
- Add test_system_session_persistence.py with 4 tests covering the exact
  production crash scenario (datetime injection → json.dumps → round-trip)
- Fix test_engine_timeout.py broken GoalExecutor module reference

GramAddict/core/bot_flow.py

@@ -307,7 +307,6 @@ def start_bot(**kwargs):
 
     try:
         bot_start_time = datetime.now()
-        configs.args.global_start_time = bot_start_time
         max_runtime = getattr(configs.args, "max_runtime_minutes", None)
 
         dopamine.global_start_time = bot_start_time

GramAddict/core/session_state.py

@@ -277,7 +277,29 @@ class SessionState:
 
 
 class SessionStateEncoder(JSONEncoder):
+    """JSON encoder for SessionState that is crash-proof against non-serializable types."""
+
+    _SAFE_TYPES = (str, int, float, bool, type(None))
+
+    @classmethod
+    def _sanitize_value(cls, value):
+        """Convert any non-JSON-serializable value to a safe string representation."""
+        if isinstance(value, cls._SAFE_TYPES):
+            return value
+        if isinstance(value, datetime):
+            return value.isoformat()
+        if isinstance(value, dict):
+            return {k: cls._sanitize_value(v) for k, v in value.items()}
+        if isinstance(value, (list, tuple)):
+            return [cls._sanitize_value(v) for v in value]
+        # Last resort: stringify unknown objects to prevent json.dump mid-write crashes
+        return str(value)
+
     def default(self, session_state: SessionState):
+        # Sanitize args dict — never trust raw __dict__, it may contain datetime or other garbage
+        raw_args = session_state.args.__dict__ if hasattr(session_state.args, "__dict__") else {}
+        safe_args = {k: self._sanitize_value(v) for k, v in raw_args.items()}
+
         return {
             "id": session_state.id,
             "total_interactions": sum(session_state.totalInteractions.values()),
@@ -291,7 +313,7 @@ class SessionStateEncoder(JSONEncoder):
             "total_scraped": session_state.totalScraped,
             "start_time": str(session_state.startTime),
             "finish_time": str(session_state.finishTime),
-            "args": session_state.args.__dict__,
+            "args": safe_args,
             "profile": {
                 "posts": session_state.my_posts_count,
                 "followers": session_state.my_followers_count,