TileMap getters for cell `TileData` ignore runtime changes stored in `runtime_tile_data_cache`

### Tested versions

4.2.1.stable

### System information

Windows 10

### Issue description

If you overwrite a tilemaps's navigation data, for example using the _tile_data_runtime_update() function, you can remove navigation from a tilemap tile. This works fine when using the Tilemap navigation. However, when you attempt to use the NavigationRegion2D (linked to the tilemap) and programmatically bake the navigation polygon using bake_navigation_polygon(), these changes are not accounted for.

### Steps to reproduce

1. Create a simple 2d scene with a TileMap and a NavigationRegion2D set to retrieve navigation information from that TileMap.
2. Attach a script to the TileMap and write the following (as an example):

```
func _use_tile_data_runtime_update(layer, coords):
if layer == 0 and coords.x == 1:
return true
return false

func _tile_data_runtime_update(layer, coords, tile_data):
tile_data.set_navigation_polygon(0, null)
return true
```

3. Attach a script to your Root node and write the following:

```
func _ready():
$TileMap.notify_runtime_tile_data_update(0)
$NavigationRegion2D.call_deferred("bake_navigation_polygon")
```

4. From the Debug Menu, enable "Visible Navigation"
5. Run the project. You will see both the tilemap's nav mesh (with column 1 excluded, as expected) and the NavigationRegion2D's nav mesh (without column 1 excluded).

### Minimal reproduction project (MRP)

[Test Navigation.zip](https://github.com/godotengine/godot/files/14610520/Test.Navigation.zip)

Reproduce the issue with the linked minimal reproduction project, starting from TileMap runtime tile data updates and NavigationRegion2D.bake_navigation_polygon(). Compare the TileMap navigation mesh with the region's baked mesh; done means runtime removal of a tile's navigation polygon is reflected in the NavigationRegion2D result.

Written by the indexing model from the issue text.