[Refactor/Chore] Milvus add_texts() returns dictionary keys instead of inserted primary keys

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### Description

### Title

Milvus add_texts() returns dictionary keys instead of inserted primary keys

### Description

MilvusClient.insert() returns a dictionary in PyMilvus 2.6.12 and 3.0.1:

{
"insert_count": 2,
"ids": ["id-1", "id-2"],
"cost": 0,
}

The current implementation passes the entire result to list.extend():

ids = self._client.insert(...)
pks.extend(ids)

Iterating over a dictionary yields its keys, so add_texts() returns:

["insert_count", "ids", "cost"]

instead of the inserted primary keys.

The existing unit test mocks insert() as returning a list, which hides the issue.

The result should be handled as follows:

result = self._client.insert(...)
pks.extend(result.get("ids", []))

The unit test should also use the actual dictionary response structure and verify
that add_texts() returns the values from the ids field.

### Motivation

_No response_

### Additional Context

_No response_