## Summary
This PR fixes#4681 by building upon the foundation laid in PR #1182
(litegraph.js). It prevents incompatible type connections when dragging
from a normal node's output to a SubgraphInputNode's occupied slot.
Before:
https://github.com/user-attachments/assets/03def938-dccc-4b2c-b65b-745abf02a13b
After:
https://github.com/user-attachments/assets/7a0a2ed4-9ecd-4147-be56-d643d448d4cb
## Background
PR #1182 implemented:
- `isValidTarget()` method in SubgraphInput/SubgraphOutput classes for
validation
- Visual feedback during drag (40% opacity for invalid targets)
- Validation at the slot level
However, there was a missing piece: while the visual feedback correctly
showed invalid targets, the actual connection would still be made when
dropped.
## Changes
This PR extends PR #1182 by adding the missing connection prevention:
1. **Added `canConnectToSubgraphInput()` method** to render link
classes:
- `MovingOutputLink`
- `ToOutputRenderLink`
- `FloatingRenderLink`
- All methods use the existing `SubgraphInput.isValidTarget()` from PR
#1182
2. **Added validation in `LinkConnector.dropOnIoNode()`**:
- Checks `canConnectToSubgraphInput()` before allowing the connection
- Logs a warning when rejecting invalid connections
- Follows the same pattern as regular node connections
3. **Added `isSubgraphInputValidDrop()` method**:
- Provides validation for hover states
- Ensures consistent validation across the UI
