fix: Remove redundant $defs from outputSchema in SmolAgentsAdapter

[Edison-A-N]

Problem

The outputSchema processing in SmolAgentsAdapter was inconsistent with inputSchema processing. While inputSchema correctly filters out redundant $defs after jsonref.replace_refs(), the outputSchema did not.

This inconsistency causes serious issues:

• Redundant $defs definitions consume excessive tokens in the LLM prompt, wasting valuable context window space
• Even GPT-5 cannot accurately understand the output schema when cluttered with these unnecessary definitions
• This prevents achieving the goal to "chain multiple tool calls and directly access structured output fields in the same code block!"

Changes

• Updated outputSchema processing to filter out $defs key after resolving references
• Now matches the pattern used for inputSchema processing, ensuring consistency across the codebase

[chahn]

Thanks a lot for catching this!
I initially put together the structured output/output schema support for mcpadapt and smolagents, and this change lines up nicely with how we handle the input schema.

Looks good to me!

@grll if it makes sense to you too, we should merge this in.

[grll]

@Edison-A-N Thanks for the fix!