fix(components): [el-tree-select] fix lazy and multiple select node

[gaoxuan-haxibiao]

1.Fix the issue where non-leaf nodes cannot be selected when using lazy and multiple together.

2. Since the original code did not handle the selection of non-leaf nodes before expanding lazy-loaded child nodes, additional processing was added to link the selection of parent and child nodes.

3. Because the time required for lazyLoad after nodeExpand cannot be predicted, the triggering time of the node-expand event in the tree has been adjusted to occur within the callback of the expand function. This change only affects the timing when using lazyLoad.

BREAKING CHANGE : no

closed closed Fixes #17232

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🧪 Playground Preview: https://element-plus.run/?pr=17903 Please comment the example via this playground if needed.

[gaoxuan-haxibiao]

When selecting a non-leaf node, the display of modelValue was designed with reference to the SHOW_CHILD strategy from Ant Design TreeSelect showCheckedStrategy. Has element-plus considered adding a showCheckedStrategy prop to the TreeSelect component?

[gaoxuan-haxibiao]

@tolking @btea Can review and merge this pr? Thank you!

[btea]

Thanks!