Return error when structured output is truncated due to max_tokens

[Param0rph]

Summary

This PR improves error handling in ResponseService.New(...) by checking the IncompleteDetails.Reason field. When the reason is "max_tokens", it now returns an explicit error.

This helps users detect truncated structured output responses before attempting to parse invalid or incomplete JSON manually.

Changes

• Added check for IncompleteDetails.Reason == "max_tokens"
• Return error with clear message when output is truncated
• Added integration test in responses_test.go using low MaxOutputTokens

Why

Previously, truncated responses would silently cause downstream json.Unmarshal errors. This change provides a more informative failure path.

Related

Resolves internal issue: unexpected JSON parse failures caused by token limits.