fix(langgraph): dedupe merged callback handlers by identity (#2706)

This fixes a double-streaming bug in certain nested graph case
scenarios.

`mergeCallbacks` concatenated `handlers` and `inheritableHandlers` while
deduping tags, so a handler inherited by both the parent and nested
config (in nested graphs) picked up an extra registration at every
nesting level with tracing on. With tracing on, a nested streamMode:
"messages" run delivered every token twice or more.

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Co-authored-by: Christian Bromann <git@bromann.dev>