fix(assistant-stream): handle tool execution errors gracefully

[ShobhitPatra]

[!IMPORTANT] Adds error handling for tool execution in unstable_runPendingTools to return error details in ToolResponse.

• Behavior:
  • Adds error handling in unstable_runPendingTools in toolResultStream.ts to catch execution errors and return a ToolResponse with isError: true and error message.
  • Updates unstable_runPendingTools to return original message if no tool calls exist.
• Tests:
  • Adds test case in toolResultStream.test.ts to verify graceful handling of tool execution errors, ensuring isError: true and correct error message in result.

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Latest commit: cbc0a8247695a7a94e8ca5b104aac287ed931634

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