[Obs AI Assistant] Improved error handling for tool response

[arturoliduena]

Closes #234193

Summary

Fixes an issue where the chat/complete stream could unexpectedly terminate when the LLM attempted to call a tool that resulted in one of the following errors:

• The tool was not available (e.g., "Tool 'execute_query' called but was not available").

• The tool was called with invalid or missing arguments.

Instead of terminating the stream, these errors are now captured and returned as part of the tool response, allowing the chat to continue normally.

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Pinging @elastic/obs-ai-assistant (Team:Obs AI Assistant)

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