collapseToolCallChunks

Chunks are grouped using the following matching logic:

• If a chunk has both an id and index, it matches chunks with the same id and index
• If a chunk has only an id, it matches chunks with the same id
• If a chunk has only an index, it matches chunks with the same index

For each group, the function:

1. Concatenates all args strings from the chunks
2. Attempts to parse the concatenated string as JSON
3. Validates that the result is a non-null object with a valid id
4. Creates either a ToolCall (if valid) or InvalidToolCall (if invalid)