Fix: Prevent PackedScene duplicate signal connection for CONNECT_PERSIST

[Andrewyuan34]

Fix: Prevent PackedScene duplicate signal connection for CONNECT_PERSIST occured in issue #103453

Issue

• When a PackedScene is instantiated, it automatically restores CONNECT_PERSIST signals.
• However, if SceneState::instantiate calls connect(), it triggers a "Signal already connected" error.

Solution

• Modified Object::connect() to check for CONNECT_PERSIST before reporting duplicate connections.
• If the signal is already connected and has CONNECT_PERSIST, it will skip reconnection instead of throwing an error.

Note:
This is my first PR to the Godot project. If there are any issues or areas for improvement, please let me know—I’m happy to make adjustments and learn from your feedback! Thank you for your patience and guidance.