Fix slot chain links for shared slots

[mzl2233]

This updates the default processor slot chain so a shared slot instance no longer has its next pointer overwritten when it is added to multiple chains. The chain now keeps its own link node for each added slot and delegates processing to the original slot, preserving each chain’s local successor. A regression test covers building two chains with the same slot instance followed by different downstream slots.

Test: mvn -pl sentinel-core -Dtest=DefaultSlotChainBuilderTest test could not be run in this environment because mvn is not installed.

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[LearningGp]