feat: add streaming to direct tool calls

[emaan-c]

Description

Direct tool calls (agent.tool.tool_name()) currently block without providing streaming events, while agent-level calls expose rich streaming from the same underlying infrastructure. This creates inconsistent developer experience and prevents building responsive UIs for long-running operations, multi-agent systems, and debugging workflows.

This adds stream() and stream_async() methods to tool calls, enabling real-time observability without recording to message history.

Resolves: #1436

Public API Changes

Tool calls now support three execution modes:

# Synchronous (unchanged - backward compatible)
result = agent.tool.fetch_data(query="users")

# NEW: Sync streaming
for event in agent.tool.fetch_data.stream(query="users"):
    print(f"Progress: {event.get('type')}")

# NEW: Async streaming
async for event in agent.tool.fetch_data.stream_async(query="users"):
    await ui.update_progress(event)

Streaming methods yield the same events as ToolExecutor._stream() without recording to message history. 100% backward compatible.

Use Cases

• Multi-agent composition: Stream sub-agent execution with full visibility
• Real-time progress: Display live indicators during long-running operations
• Debugging: Inspect execution flow without polluting conversation history

Related Issues

#1436

Documentation PR

No documentation PR needed.

Type of Change

New feature

Testing

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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