Expose failure_error_function in Agent.as_tool

[Nekowaii]

Summary

Expose failure_error_function in Agent.as_tool() and pass it through to @function_tool(...).

Motivation

When an Agent is wrapped via as_tool(), exceptions raised during the nested Runner.run(...) (including failures triggered by hosted tools like Code Interpreter, file/web search, etc.) are currently converted into a generic tool error string by the default function_tool error handler. This makes it difficult to:

• handle failures programmatically in the caller, and/or
• report failures via standard exception-based observability.

This change keeps the default behavior unchanged, but allows opting into exception propagation via failure_error_function=None.

[seratch]

Sorry, can you resolve the conflicts? Your changes here look good to me.

[Nekowaii]

Done. I rebased and fixed the conflicts. Thanks.