Fix Dictionary serialization in JSON sink for MongoDB nested documents

[Copilot]

MongoDB's BsonTypeMapper.MapToDotNetValue() converts nested BsonDocuments to Dictionary<string, object?>, which were serialized as "System.Collections.Generic.Dictionary2[System.String,System.Object]"` instead of proper JSON objects.

Changes

DataItemJsonConverter.cs

• Added IDictionary<string, object?> check before fallback to .ToString()
• Converts dictionaries to DictionaryDataItem for proper nested object serialization
• Handles both field-level dictionaries and dictionaries within arrays

Tests

• Unit tests for dictionary field values and arrays of dictionaries
• Integration test validating complete MongoDB export scenario

Example

Before:

{
  "_id": "some_id",
  "content": ["System.Collections.Generic.Dictionary`2[System.String,System.Object]"]
}

After:

{
  "_id": {"$oid": "some_id"},
  "content": [{"text": "a message text", "type": "text"}]
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Nested elements not supported in JSON Sink</issue_title>
<issue_description>Hi

I am trying to eksport from MongoDB to JSON.

An element like the following:

{
  "_id": {
    "$oid": "some_id"
  },
  "thread_id": "thread_id",
  "content": [
    {
      "text": "a message text",
      "type": "text"
    }
  ],
  "role": "user"
}

Is in JSON exported as:

{
    "_id": "some_id",
    "thread_id": "thread_id",
    "content": [
        "System.Collections.Generic.Dictionary`2[System.String,System.Object]"
    ],
    "role": "user"
}

So while "content" is perserved as a list, its contents are gone.

</issue_description>

Comments on the Issue (you are @copilot in this section)

@philnach I was observing something similar. assigning to myself for investigation.

• Fixes Nested elements not supported in JSON Sink #153

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