fix: do not synthesize exception for cancelled tools

[Gastly]

Description

After #2046 , setting cancel_tool on a BeforeToolCallEvent now populates AfterToolCallEvent.exception with a synthesized exception which cannot be overwritten, causing tool cancellations to appear as failures to downstream hooks.

Cancellation is a deliberate action, not a failure. AfterToolCallEvent.cancel_message already exists to identify cancelled tools; hooks can use it to distinguish cancellations from genuine exceptions without needing a fake exception object. Additionally, tool result status remains as error for cancellations.

Span observability is preserved: StatusCode.ERROR is now set by inspecting tool_result["status"] directly rather than relying on a synthesized exception.

Public API Changes

AfterToolCallEvent.exception is once again None for cancelled tools. cancel_message remains the correct field to detect cancellations.

Related Issues

#2046

Documentation PR

No documentation changes needed.

Type of Change

Bug fix

Testing

• [x ] I ran hatch run prepare

Checklist

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

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