Discard handler return value in exception handling to prevent polluting raw_response

[TheRealNeil]

Discard handler return value in exception handling to prevent polluting raw_response

Fixes #305

Summary

• Add explicit nil return after rescue_with_handler in with_exception_handling to prevent the handler's return value leaking into raw_response

Root cause

When an exception occurs during api_prompt_execute, with_exception_handling returns whatever the rescue handler returns (often a truthy value like a String from broadcast_error). This becomes raw_response in resolve_prompt, which then causes a secondary NoMethodError: undefined method 'usage' for an instance of String — swallowing the real error.

Fix

Return nil explicitly after rescue_with_handler(exception) || raise. This ensures raw_response is always nil when an exception has been handled, preventing the secondary error while still letting the agent's rescue_from chain handle the original exception.

Test plan

• Trigger a provider exception with a rescue_from handler that returns a truthy value and confirm no secondary NoMethodError on usage
• Confirm unhandled exceptions still re-raise as expected
• Confirm rescue_from handlers are still invoked correctly