Fix json::update() with merge_objects==true error

[DanielHwangbo]

Description

This PR fixes a crash when using update() on an ordered_json object when JSON_DIAGNOSTICS is enabled.

The Issue

When update() inserts new elements into an ordered_json object, the underlying std::vector may reallocate. This invalidates the m_parent pointers of existing children, causing assert_invariant to fail immediately because the children point to the old memory address.

The Fix

I have updated the update() function to call set_parents() after insertion operations when diagnostics are enabled. This ensures that if the container moves in memory, all children are correctly updated to point to the new parent address.

Reproduction Code

#define JSON_DIAGNOSTICS 1
#include "nlohmann/json.hpp"
#include <iostream>

int main() {
    using json = nlohmann::ordered_json;
    json j1 = { {"numbers", {{"one", 1}}} };
    json j2 = { {"numbers", {{"two", 2}}}, {"string", "t"} };

    // This triggers reallocation and assertion failure without the fix
    j1.update(j2, true); 
    std::cout << j1.dump() << std::endl;
}

[coveralls]

coverage: 99.176% (-0.02%) from 99.191%
when pulling 0d1eb84 on DanielHwangbo:issue4813
into a0e9fb1 on nlohmann:develop.

[github-actions]

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[nlohmann]

Maybe related: #4813