fix: handle malformed JSON in structChat parsed response

[JacobiusMakes]

Summary

convertToParsedChatCompletionResponse in src/extra/structChat.ts crashes with an unhandled SyntaxError when the model returns malformed JSON content (common when finish_reason is "length" due to token limit truncation).

Bug: The JSON.parse() call on line 127 is unguarded. While responseFormat.safeParse() correctly uses Zod's safe parsing, the preceding JSON.parse() throws on invalid JSON, propagating an uncaught exception to the caller.

Secondary issue: When a choice has null, undefined, or array content, the choice was silently dropped from the output (the inner if had no else branch), causing the response to have fewer choices than expected.

Changes

• Wrap JSON.parse() in a try/catch so malformed JSON gracefully sets parsed to undefined instead of throwing
• Add an else branch to preserve choices with non-string content (null/undefined/array) with parsed: undefined
• Add 5 new test cases covering: malformed JSON, schema validation failure, null content, empty choices, undefined choices

Test plan

• All 8 structChat tests pass (3 existing + 5 new)
• Full test suite passes (39/39 tests)
• Verify behavior with a real truncated response from the API