fix: Track connection closed state internally

The is_connected property was relying on ping() to determine if a connection
was closed, but MySQLdb's ping() may succeed even after close() is called.

Now tracks connection state with _is_closed flag that is:
- Set to True in __init__ (before connect)
- Set to False after successful connect()
- Set to True in close()
- Checked first in is_connected before attempting ping()

Fixes test_connection_context_manager, test_connection_context_manager_exception,
and test_close failures.

Author: dimitri-yatsenko

src/datajoint/connection.py

@@ -177,6 +177,7 @@ def __init__(
         self.init_fun = init_fun
         self._conn = None
         self._query_cache = None
+        self._is_closed = True  # Mark as closed until connect() succeeds
 
         # Select adapter based on configured backend
         backend = config["database.backend"]
@@ -228,6 +229,7 @@ def connect(self) -> None:
                     )
                 else:
                     raise
+        self._is_closed = False  # Mark as connected after successful connection
 
     def set_query_cache(self, query_cache: str | None = None) -> None:
         """
@@ -255,7 +257,9 @@ def purge_query_cache(self) -> None:
 
     def close(self) -> None:
         """Close the database connection."""
-        self._conn.close()
+        if self._conn is not None:
+            self._conn.close()
+        self._is_closed = True
 
     def __enter__(self) -> "Connection":
         """
@@ -329,9 +333,12 @@ def is_connected(self) -> bool:
         bool
             True if connected.
         """
+        if self._is_closed:
+            return False
         try:
             self.ping()
         except:
+            self._is_closed = True
             return False
         return True