fix: accumulate streaming tool call arguments across SSE chunks

OpenAI's SSE streaming sends tool calls incrementally - the first chunk
has the id and function name, subsequent chunks contain argument fragments.
Previously each chunk was treated as a complete tool call, causing invalid
JSON arguments.

- Add ToolCallDelta type with index for partial tool call data
- Add StreamToolCallDelta chunk type
- Track tool calls by index in IntMap accumulator
- Merge argument fragments across chunks via mergeToolCallDelta
- Build final ToolCall objects from accumulator when stream ends
- Handle new StreamToolCallDelta in Engine.hs pattern match

0 files changed, 0 insertions, 0 deletions