fix: propagate tool exceptions to spans so StatusCode.ERROR is set correctly

[mattdai01]

When a tool fails, the exception was being dropped before reaching end_tool_call_span, causing all tool spans to get StatusCode.OK even on errors. This threads the original exception through ToolResultEvent yields and into the tracer, and creates exceptions for cancel/unknown tool paths that previously had none.

Fixes #2016

Description

When a tool raises an exception, Strands catches it and converts it to a ToolResult dict with status='error'. The original exception object was dropped at the ToolResultEvent yield points in _executor.py, so end_tool_call_span never received it and always set StatusCode.OK. This breaks observability backends like Langfuse that rely on span status codes for error detection.

This PR fixes the root cause by:

• Threading original exceptions through ToolResultEvent yields and into end_tool_call_span so tool spans correctly get StatusCode.ERROR
• Creating exceptions for cancel and unknown-tool error paths that previously produced status: "error" results without an exception object

PR #2027 proposed synthesizing an Exception from error text in tracer.py. This PR instead fixes the root cause — the exception was already available but simply not threaded through. This preserves the original exception type and traceback.

Related Issues

Fixes #2016

Documentation PR

N/A

Type of Change

Bug fix

Testing

• I ran hatch run prepare
• Full test suite: 2488 passed, 0 failures
• New tests:
  • test_end_tool_call_span_with_error — explicit error sets StatusCode.ERROR and record_exception
  • test_executor_stream_with_trace_error — exception flows from tool to tracer
  • test_executor_stream_error_preserves_exception — ToolResultEvent.exception is set
  • test_executor_stream_unknown_tool_has_exception — unknown tool path
  • test_executor_stream_cancel_has_exception — cancel path

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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