3DTILES_multiple_contents no supports more than 6 contents

3DTILES_multiple_contents no supports more than 6 contents, except change the cesium static setting: `Cesium.RequestScheduler.maximumRequestsPerServer = 8;`

, or the tile cannot load. But this extension schema [link ](https://github.com/CesiumGS/3d-tiles/blob/main/extensions/3DTILES_multiple_contents/schema/tile.3DTILES_multiple_contents.schema.json)no contains this limitaiton.

`"3DTILES_multiple_contents": { // 7 contents
"contents": [
{
"uri": "B_0.b3dm"
},
{
"uri": "I_0_0.i3dm"
},
{
"uri": "I_0_1.i3dm"
},
{
"uri": "I_0_2.i3dm"
},
{
"uri": "I_0_3.i3dm"
},
{
"uri": "I_0_4.i3dm"
},
{
"uri": "I_0_5.i3dm"
}
]
}`

Start by reproducing the 3DTILES_multiple_contents example with seven content URIs and compare it with Cesium.RequestScheduler.maximumRequestsPerServer = 8. Read the linked 3DTILES_multiple_contents schema and trace the tile-loading behavior involved. Done means the supported content limit and the required scheduler behavior are clarified and reflected consistently in the implementation or schema.

Written by the indexing model from the issue text.