fix(core): prevent tool call chunks from merging incorrectly in AIMes…

[christian-bromann]

Description

This PR fixes a bug in AIMessageChunk.concat() where tool call chunks with the same index were being incorrectly merged instead of preserved as separate tool calls.

Problem

When using models like Ollama that return multiple tool calls in chunks, the _mergeLists function was merging tool call chunks that had the same index property. This resulted in malformed tool calls with concatenated properties:

// Before fix - incorrectly merged:
{
  name: "add_new_taskadd_ideas",
  args: '{"tasks":["buy tomatoes","help child with math"]}{"ideas":["read about Angular 19 updates"]}',
  id: "9fb5c937-6944-4173-84be-ad1caee1cedd1c9d8514-874a-4d8a-a9a9-604b9d831b60"
}

// After fix - correctly preserved as separate calls:
[
  {
    name: "add_new_task",
    args: '{"tasks":["buy tomatoes","help child with math"]}',
    id: "9fb5c937-6944-4173-84be-ad1caee1cedd"
  },
  {
    name: "add_ideas", 
    args: '{"ideas":["read about Angular 19 updates"]}',
    id: "5abf542e-87f3-4899-87c6-8f7d9cb6a28d"
  }
]

Solution

• Replaced _mergeLists() with simple array concatenation using the spread operator
• Added comprehensive test case to verify tool call chunks remain separate during concatenation
• Removed unused _mergeLists import

Impact

• ✅ Multiple tool calls now work correctly with streaming providers like Ollama
• ✅ Tool call arguments remain valid JSON and can be parsed properly
• ✅ No more invalid_tool_calls due to malformed merged data
• ✅ Existing functionality preserved (all tests pass)

Testing

• Added specific test case for the reported scenario
• All existing AIMessageChunk tests continue to pass
• Verified fix works with the exact example from the issue report

Fixes #8276

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