fix(ai): serialize undefined historical tool input - #48863

Summary

• Serialize an explicit undefined historical tool input as {} in OpenAI Chat and shared Responses tool-call lowering.
• Preserve defined inputs, including null, arrays, strings, numbers, and objects.

Schema and failure verification

ToolCallPart.input is Schema.Unknown. An explicit input: undefined passes both the part schema and decoding into the canonical Message class. Omitting the required input property entirely is rejected by the part schema.

Before this change, an otherwise schema-valid message containing undefined input failed synchronously in JSON encoding, before making an HTTP request. The new tests decode the message through the real schema, compile it successfully, assert arguments: "{}", and verify the canonical input remains unchanged.

Live verification

Requests containing undefined historical input plus a matching error result were sent through the actual native compiler and LLMClient.generate streaming path using gpt-4.1-mini on both OpenAI Chat and Responses. Both completed normally and returned the exact fixture error code INPUT_MISSING, with no generated tool calls. No model-requested tools were executed.

Checks

• bun test test/provider/missing-tool-input.test.ts test/provider/openai-chat.test.ts test/provider/openai-responses.test.ts --timeout 30000 --only-failures — 215 passed.
• bun typecheck from packages/ai — passed.
• Formatting and git diff --check — passed.