Add streaming tool result events

[ai-fanatic]

Summary

• add streaming tool output support (acall_stream) with async generator handling in FunctionTool
• emit preliminary ToolCallResult events and ignore them during aggregation to avoid early completion
• add test coverage for streaming tool outputs

Testing

• uv run pre-commit run --show-diff-on-failure --files <changed files> (Windows; equivalent to make lint)
• uv run -- pytest llama-index-core/tests/agent/workflow/test_tool_streaming.py

AI Assistance

I used ChatGPT/Codex to help draft the changes. I reviewed the code and verified behavior via the lint and test commands above.

Issue

Relates to #20409

[logan-markewich]

I think some of these changes weren't intended to be in this PR?

[logan-markewich]

Why only acall? Why not the sync version as well? Had a similar comment on another user that attempted this PR
#20552

[ai-fanatic]

Thanks for the review.

• Unintended changes: I rebased the branch onto upstream/main to drop unrelated structured‑output commits, then force‑pushed. The PR now only includes streaming tool result changes.
• Sync version: added sync streaming support (FunctionTool.call_stream, BaseTool.call_stream, BaseToolAsyncAdapter.acall_stream) and treat sync generator functions as streaming; also added a sync generator test alongside the async one.

Checks run:

• uv run pre-commit run --show-diff-on-failure --files <changed files>
• uv run -- pytest llama-index-core/tests/agent/workflow/test_tool_streaming.py

[logan-markewich]

llama_index/core/tools/function_tool.py:437: error: "Awaitable[Any]" has no attribute "__aiter__"; maybe "__await__"? (not async iterable)  [attr-defined]
llama_index/core/tools/function_tool.py:440: error: No overload variant of "iter" matches argument type "Awaitable[Any]"  [call-overload]
llama_index/core/tools/function_tool.py:440: note: Possible overload variants:
llama_index/core/tools/function_tool.py:440: note:     def [_SupportsNextT: SupportsNext[Any]] iter(SupportsIter[_SupportsNextT], /) -> _SupportsNextT
llama_index/core/tools/function_tool.py:440: note:     def [_T] iter(_GetItemIterable[_T], /) -> Iterator[_T]
llama_index/core/tools/function_tool.py:440: note:     def [_T] iter(Callable[[], _T | None], None, /) -> Iterator[_T]
llama_index/core/tools/function_tool.py:440: note:     def [_T] iter(Callable[[], _T], object, /) -> Iterator[_T]

[ai-fanatic]

Fixed the mypy errors in FunctionTool.acall_stream by explicitly typing raw_output as Any before checking awaitable/iterable branches. Pushed commit 4a3a178 to feature/native-structured-output-18412.